diff --git a/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttHandler.cs b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttHandler.cs
index ae290fc..1b43f32 100644
--- a/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttHandler.cs
+++ b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttHandler.cs
@@ -91,6 +91,16 @@ internal sealed class MqttHandler
     /// </summary>
     public bool DowngradeQoS2Sub { get; set; }
 
+    // ------------------------------------------------------------------
+    // QoS 2 in-memory pending store (full JetStream persistence in Task 6)
+    // ------------------------------------------------------------------
+
+    /// <summary>
+    /// In-memory store for QoS 2 inbound messages awaiting PUBREL.
+    /// Keyed by packet identifier. Replaces JetStream $MQTT_qos2in stream.
+    /// </summary>
+    public Dictionary<ushort, MqttPublishInfo> QoS2Pending { get; } = new();
+
     // ------------------------------------------------------------------
     // Parse state (used by the read-loop MQTT byte-stream parser)
     // ------------------------------------------------------------------
diff --git a/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttPacketHandlers.cs b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttPacketHandlers.cs
index 366df7a..0f93b51 100644
--- a/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttPacketHandlers.cs
+++ b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttPacketHandlers.cs
@@ -313,21 +313,146 @@ internal static class MqttPacketHandlers
 
     /// <summary>
     /// Processes an inbound MQTT PUBLISH packet.
-    /// QoS 0: routes immediately via NATS pub/sub. QoS 1/2: deferred to Task 5.
+    /// QoS 0: routes immediately. QoS 1: deliver + PUBACK. QoS 2: store + PUBREC.
     /// Mirrors Go <c>mqttProcessPub()</c>.
     /// </summary>
     public static Exception? ProcessPub(ClientConnection c, MqttPublishInfo pp)
     {
-        if (pp.Qos == 0)
+        var payload = pp.Msg ?? [];
+
+        switch (pp.Qos)
         {
-            // QoS 0: immediate delivery via internal routing.
-            var payload = pp.Msg ?? [];
-            c.ProcessInboundClientMsg(payload);
-            return null;
+            case 0:
+                c.ProcessInboundClientMsg(payload);
+                return null;
+
+            case 1:
+                // QoS 1: deliver immediately, then send PUBACK.
+                c.ProcessInboundClientMsg(payload);
+                EnqueuePubResponse(c, MqttPacket.PubAck, pp.Pi);
+                return null;
+
+            case 2:
+                // QoS 2: store message pending PUBREL, then send PUBREC.
+                // Full JetStream persistence deferred to Task 6 — uses in-memory store.
+                lock (c)
+                {
+                    c.Mqtt!.QoS2Pending[pp.Pi] = pp;
+                }
+                EnqueuePubResponse(c, MqttPacket.PubRec, pp.Pi);
+                return null;
+
+            default:
+                return new InvalidOperationException($"invalid QoS: {pp.Qos}");
+        }
+    }
+
+    // =========================================================================
+    // PUBACK / PUBREC / PUBREL / PUBCOMP handling
+    // =========================================================================
+
+    /// <summary>
+    /// Parses a packet-identifier-only packet (PUBACK, PUBREC, PUBREL, PUBCOMP).
+    /// Returns (pi, error). Mirrors Go <c>mqttParsePIPacket()</c>.
+    /// </summary>
+    public static (ushort pi, Exception? err) ParsePiPacket(MqttReader r)
+    {
+        ushort pi;
+        try { pi = r.ReadUInt16("packet identifier"); }
+        catch (Exception ex) { return (0, ex); }
+        if (pi == 0)
+            return (0, new InvalidOperationException("packet identifier must not be 0"));
+        return (pi, null);
+    }
+
+    /// <summary>
+    /// Processes an inbound PUBREL packet (QoS 2 phase 2).
+    /// Retrieves the stored QoS 2 message, delivers it, and sends PUBCOMP.
+    /// Mirrors Go <c>mqttProcessPubRel()</c>.
+    /// </summary>
+    public static Exception? ProcessPubRel(ClientConnection c, ushort pi)
+    {
+        // Always send PUBCOMP, even if message not found (idempotency).
+        MqttPublishInfo? pp;
+        lock (c)
+        {
+            c.Mqtt!.QoS2Pending.Remove(pi, out pp);
         }
 
-        // QoS 1/2: deferred to Task 5 (JetStream integration).
-        return new NotImplementedException($"PUBLISH QoS {pp.Qos} not yet implemented");
+        // Send PUBCOMP.
+        EnqueuePubResponse(c, MqttPacket.PubComp, pi);
+
+        // Deliver the stored message if found.
+        if (pp != null)
+        {
+            var payload = pp.Msg ?? [];
+            c.ProcessInboundClientMsg(payload);
+        }
+
+        return null;
+    }
+
+    /// <summary>
+    /// Processes an inbound PUBACK (client acknowledges QoS 1 message from server).
+    /// Removes from pending tracking. Mirrors Go <c>mqttProcessPubAck()</c>.
+    /// </summary>
+    public static Exception? ProcessPubAck(ClientConnection c, ushort pi)
+    {
+        lock (c)
+        {
+            c.Mqtt!.Pending.Remove(pi);
+        }
+        return null;
+    }
+
+    /// <summary>
+    /// Processes an inbound PUBREC (client acknowledges QoS 2 message from server, phase 1).
+    /// Transitions to PUBREL phase. Mirrors Go <c>mqttProcessPubRec()</c>.
+    /// </summary>
+    public static Exception? ProcessPubRec(ClientConnection c, ushort pi)
+    {
+        // In the full implementation, this would store a PUBREL in JetStream.
+        // For now, send PUBREL immediately and remove from pending.
+        lock (c)
+        {
+            c.Mqtt!.Pending.Remove(pi);
+        }
+        EnqueuePubResponse(c, MqttPacket.PubRel, pi);
+        return null;
+    }
+
+    /// <summary>
+    /// Processes an inbound PUBCOMP (client acknowledges PUBREL, QoS 2 complete).
+    /// Mirrors Go <c>mqttProcessPubComp()</c>.
+    /// </summary>
+    public static Exception? ProcessPubComp(ClientConnection c, ushort pi)
+    {
+        // Final cleanup — remove any tracking for this PI.
+        lock (c)
+        {
+            c.Mqtt!.Pending.Remove(pi);
+        }
+        return null;
+    }
+
+    /// <summary>
+    /// Enqueues a PUBACK/PUBREC/PUBREL/PUBCOMP response packet.
+    /// All four share the same 4-byte format: [type] [0x02] [PI high] [PI low].
+    /// PUBREL has bit 1 set in byte 0 per MQTT spec [3.6.1-1].
+    /// Mirrors Go <c>mqttEnqueuePubResponse()</c>.
+    /// </summary>
+    public static void EnqueuePubResponse(ClientConnection c, byte packetType, ushort pi)
+    {
+        var b0 = packetType;
+        // PUBREL requires fixed header bits 0010 per MQTT spec.
+        if (packetType == MqttPacket.PubRel)
+            b0 |= 0x02;
+
+        ReadOnlySpan<byte> packet = [b0, 0x02, (byte)(pi >> 8), (byte)(pi & 0xFF)];
+        lock (c)
+        {
+            c.EnqueueProto(packet);
+        }
     }
 
     // =========================================================================
diff --git a/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttParser.cs b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttParser.cs
index 007fd90..b4038ae 100644
--- a/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttParser.cs
+++ b/dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttParser.cs
@@ -92,24 +92,36 @@ internal static class MqttParser
                     break;
 
                 case MqttPacket.PubAck:
-                    // TODO: Task 5 — process PUBACK
-                    err = new NotImplementedException("PUBACK not yet implemented");
+                {
+                    var (ackPi, ackErr) = MqttPacketHandlers.ParsePiPacket(r);
+                    if (ackErr != null) { err = ackErr; break; }
+                    err = MqttPacketHandlers.ProcessPubAck(c, ackPi);
                     break;
+                }
 
                 case MqttPacket.PubRec:
-                    // TODO: Task 5 — process PUBREC
-                    err = new NotImplementedException("PUBREC not yet implemented");
+                {
+                    var (recPi, recErr) = MqttPacketHandlers.ParsePiPacket(r);
+                    if (recErr != null) { err = recErr; break; }
+                    err = MqttPacketHandlers.ProcessPubRec(c, recPi);
                     break;
+                }
 
                 case MqttPacket.PubRel:
-                    // TODO: Task 5 — process PUBREL
-                    err = new NotImplementedException("PUBREL not yet implemented");
+                {
+                    var (relPi, relErr) = MqttPacketHandlers.ParsePiPacket(r);
+                    if (relErr != null) { err = relErr; break; }
+                    err = MqttPacketHandlers.ProcessPubRel(c, relPi);
                     break;
+                }
 
                 case MqttPacket.PubComp:
-                    // TODO: Task 5 — process PUBCOMP
-                    err = new NotImplementedException("PUBCOMP not yet implemented");
+                {
+                    var (compPi, compErr) = MqttPacketHandlers.ParsePiPacket(r);
+                    if (compErr != null) { err = compErr; break; }
+                    err = MqttPacketHandlers.ProcessPubComp(c, compPi);
                     break;
+                }
 
                 case MqttPacket.Sub:
                     var (subPi, subFilters, subErr) = MqttPacketHandlers.ParseSubsOrUnsubs(r, b, pl, isSub: true);
diff --git a/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/Mqtt/MqttPubSubTests.cs b/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/Mqtt/MqttPubSubTests.cs
index ede803e..9d185bb 100644
--- a/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/Mqtt/MqttPubSubTests.cs
+++ b/dotnet/tests/ZB.MOM.NatsNet.Server.Tests/Mqtt/MqttPubSubTests.cs
@@ -174,14 +174,14 @@ public sealed class MqttPubSubTests
     }
 
     [Fact]
-    public void Parser_PublishQoS1_ShouldReturnNotImplemented()
+    public void Parser_PublishQoS1_ShouldSendPubAck()
     {
         var c = CreateConnectedMqttClient();
 
-        // PUBLISH QoS 1: type=0x32, topic="t", PI=1, payload="x"
+        // PUBLISH QoS 1: type=0x32, topic="t", PI=5, payload="x"
         var data = new List<byte>();
         data.Add(0x00); data.Add(0x01); data.Add((byte)'t'); // topic
-        data.Add(0x00); data.Add(0x01); // PI = 1
+        data.Add(0x00); data.Add(0x05); // PI = 5
         data.Add((byte)'x'); // payload
 
         var buf = new List<byte>();
@@ -190,8 +190,146 @@ public sealed class MqttPubSubTests
         buf.AddRange(data);
 
         var err = MqttParser.Parse(c, buf.ToArray(), buf.Count);
-        err.ShouldNotBeNull();
-        err.ShouldBeOfType<NotImplementedException>();
+        err.ShouldBeNull();
+
+        // Verify PUBACK: [0x40] [0x02] [PI high] [PI low]
+        var ms = GetStream(c);
+        var written = ms.ToArray();
+        written.Length.ShouldBe(4);
+        written[0].ShouldBe(MqttPacket.PubAck); // 0x40
+        written[1].ShouldBe((byte)0x02);
+        written[2].ShouldBe((byte)0x00); // PI high
+        written[3].ShouldBe((byte)0x05); // PI low
+    }
+
+    [Fact]
+    public void Parser_PublishQoS2_ShouldSendPubRec()
+    {
+        var c = CreateConnectedMqttClient();
+
+        // PUBLISH QoS 2: type=0x34, topic="t", PI=10, payload="y"
+        var data = new List<byte>();
+        data.Add(0x00); data.Add(0x01); data.Add((byte)'t'); // topic
+        data.Add(0x00); data.Add(0x0A); // PI = 10
+        data.Add((byte)'y'); // payload
+
+        var buf = new List<byte>();
+        buf.Add((byte)(MqttPacket.Pub | MqttPubFlag.QoS2)); // 0x34
+        buf.Add((byte)data.Count);
+        buf.AddRange(data);
+
+        var err = MqttParser.Parse(c, buf.ToArray(), buf.Count);
+        err.ShouldBeNull();
+
+        // Verify PUBREC: [0x50] [0x02] [PI high] [PI low]
+        var ms = GetStream(c);
+        var written = ms.ToArray();
+        written.Length.ShouldBe(4);
+        written[0].ShouldBe(MqttPacket.PubRec); // 0x50
+        written[1].ShouldBe((byte)0x02);
+        written[2].ShouldBe((byte)0x00); // PI high
+        written[3].ShouldBe((byte)0x0A); // PI low
+
+        // Message should be stored in QoS2Pending.
+        c.Mqtt!.QoS2Pending.ShouldContainKey((ushort)10);
+    }
+
+    [Fact]
+    public void Parser_QoS2_FullHandshake_PubRecPubRelPubComp()
+    {
+        var c = CreateConnectedMqttClient();
+
+        // Step 1: PUBLISH QoS 2 → PUBREC
+        var pubData = new List<byte>();
+        pubData.Add(0x00); pubData.Add(0x01); pubData.Add((byte)'t');
+        pubData.Add(0x00); pubData.Add(0x07); // PI = 7
+        pubData.AddRange(Encoding.UTF8.GetBytes("qos2msg"));
+
+        var pubBuf = new List<byte>();
+        pubBuf.Add((byte)(MqttPacket.Pub | MqttPubFlag.QoS2));
+        pubBuf.Add((byte)pubData.Count);
+        pubBuf.AddRange(pubData);
+
+        var err = MqttParser.Parse(c, pubBuf.ToArray(), pubBuf.Count);
+        err.ShouldBeNull();
+        c.Mqtt!.QoS2Pending.ShouldContainKey((ushort)7);
+
+        // Reset stream to capture only PUBCOMP.
+        var ms = GetStream(c);
+        ms.SetLength(0);
+
+        // Step 2: PUBREL from client → PUBCOMP
+        // PUBREL: [0x62] [0x02] [PI high] [PI low]
+        var pubrelBuf = new byte[] { 0x62, 0x02, 0x00, 0x07 };
+        err = MqttParser.Parse(c, pubrelBuf, pubrelBuf.Length);
+        err.ShouldBeNull();
+
+        // Verify PUBCOMP was sent.
+        var written = ms.ToArray();
+        written.Length.ShouldBe(4);
+        written[0].ShouldBe(MqttPacket.PubComp); // 0x70
+        written[1].ShouldBe((byte)0x02);
+        written[2].ShouldBe((byte)0x00);
+        written[3].ShouldBe((byte)0x07);
+
+        // Message should be removed from QoS2Pending.
+        c.Mqtt.QoS2Pending.ShouldNotContainKey((ushort)7);
+    }
+
+    [Fact]
+    public void Parser_PubAck_ShouldRemoveFromPending()
+    {
+        var c = CreateConnectedMqttClient();
+
+        // Pre-populate pending.
+        c.Mqtt!.Pending[(ushort)3] = null;
+        c.Mqtt.Pending.ShouldContainKey((ushort)3);
+
+        // PUBACK: [0x40] [0x02] [0x00] [0x03]
+        var buf = new byte[] { 0x40, 0x02, 0x00, 0x03 };
+        var err = MqttParser.Parse(c, buf, buf.Length);
+        err.ShouldBeNull();
+
+        c.Mqtt.Pending.ShouldNotContainKey((ushort)3);
+    }
+
+    [Fact]
+    public void Parser_PubRec_ShouldSendPubRel()
+    {
+        var c = CreateConnectedMqttClient();
+
+        // Pre-populate pending.
+        c.Mqtt!.Pending[(ushort)9] = null;
+
+        // PUBREC: [0x50] [0x02] [0x00] [0x09]
+        var buf = new byte[] { 0x50, 0x02, 0x00, 0x09 };
+        var err = MqttParser.Parse(c, buf, buf.Length);
+        err.ShouldBeNull();
+
+        // Should have sent PUBREL: [0x62] [0x02] [0x00] [0x09]
+        var ms = GetStream(c);
+        var written = ms.ToArray();
+        written.Length.ShouldBe(4);
+        written[0].ShouldBe((byte)0x62); // PUBREL with bit 1 set
+        written[3].ShouldBe((byte)0x09);
+
+        // Pending should be cleared.
+        c.Mqtt.Pending.ShouldNotContainKey((ushort)9);
+    }
+
+    [Fact]
+    public void Parser_PubComp_ShouldRemoveFromPending()
+    {
+        var c = CreateConnectedMqttClient();
+
+        c.Mqtt!.Pending[(ushort)15] = null;
+
+        // PUBCOMP: [0x70] [0x02] [0x00] [0x0F]
+        var buf = new byte[] { 0x70, 0x02, 0x00, 0x0F };
+        var err = MqttParser.Parse(c, buf, buf.Length);
+        err.ShouldBeNull();
+
+        c.Mqtt.Pending.ShouldNotContainKey((ushort)15);
     }
 
     // =========================================================================
diff --git a/reports/current.md b/reports/current.md
index 42ba672..21756a4 100644
--- a/reports/current.md
+++ b/reports/current.md
@@ -1,6 +1,6 @@
 # NATS .NET Porting Status Report
 
-Generated: 2026-03-01 21:04:38 UTC
+Generated: 2026-03-01 21:08:29 UTC
 
 ## Modules (12 total)