diff --git a/SteamKit2/SteamKit2/Base/ClientMsg.cs b/SteamKit2/SteamKit2/Base/ClientMsg.cs
index ceef49be3..689cc556e 100644
--- a/SteamKit2/SteamKit2/Base/ClientMsg.cs
+++ b/SteamKit2/SteamKit2/Base/ClientMsg.cs
@@ -109,9 +109,12 @@ internal ClientMsgProtobuf( EMsg eMsg, int payloadReserve = 64 )
         public ClientMsgProtobuf( IPacketMsg msg )
             : this( msg.GetMsgTypeWithNullCheck( nameof(msg) ) )
         {
-            DebugLog.Assert(msg.IsProto, "ClientMsgProtobuf", "ClientMsgProtobuf used for non-proto message!");
+            if ( !( msg is PacketClientMsgProtobuf packetMsgProto ) )
+            {
+                throw new InvalidDataException( "ClientMsgProtobuf used for non-proto message!" );
+            }
 
-            Deserialize( msg.GetData() );
+            Header = packetMsgProto.Header;
         }
 
 
@@ -123,21 +126,6 @@ public override byte[] Serialize()
         {
             throw new NotSupportedException( "ClientMsgProtobuf is for reading only. Use ClientMsgProtobuf<T> for serializing messages." );
         }
-
-        /// <summary>
-        /// Initializes this client message by deserializing the specified data.
-        /// </summary>
-        /// <param name="data">The data representing a client message.</param>
-        public override void Deserialize( byte[] data )
-        {
-            if ( data == null )
-            {
-                throw new ArgumentNullException( nameof(data) );
-            }
-
-            using MemoryStream ms = new MemoryStream( data );
-            Header.Deserialize( ms );
-        }
     }
 
     /// <summary>
@@ -147,10 +135,16 @@ public override void Deserialize( byte[] data )
     public sealed class ClientMsgProtobuf<TBody> : ClientMsgProtobuf
         where TBody : IExtensible, new()
     {
+        private TBody _body;
+
         /// <summary>
         /// Gets the body structure of this message.
         /// </summary>
-        public TBody Body { get; private set; }
+        public TBody Body
+        {
+            get => _body;
+            set => _body = value ?? throw new ArgumentNullException( nameof( value ) );
+        }
 
 
         /// <summary>
@@ -162,7 +156,7 @@ public sealed class ClientMsgProtobuf<TBody> : ClientMsgProtobuf
         public ClientMsgProtobuf( EMsg eMsg, int payloadReserve = 64 )
             : base( payloadReserve )
         {
-            Body = new TBody();
+            _body = new TBody();
 
             // set our emsg
             Header.Msg = eMsg;
@@ -190,9 +184,27 @@ public ClientMsgProtobuf( EMsg eMsg, MsgBase<MsgHdrProtoBuf> msg, int payloadRes
         public ClientMsgProtobuf( IPacketMsg msg )
             : this( msg.GetMsgTypeWithNullCheck( nameof(msg) ) )
         {
-            DebugLog.Assert( msg.IsProto, "ClientMsgProtobuf<>", $"ClientMsgProtobuf<{typeof(TBody).FullName}> used for non-proto message!" );
+            if ( !( msg is PacketClientMsgProtobuf packetMsg ) )
+            {
+                throw new InvalidDataException( $"ClientMsgProtobuf<{typeof(TBody).FullName}> used for non-proto message!" );
+            }
+
+            Header = packetMsg.Header;
 
-            Deserialize( msg.GetData() );
+            var data = packetMsg.GetData();
+            var offset = (int)packetMsg.BodyOffset;
+            using MemoryStream ms = new MemoryStream( data, offset, data.Length - offset );
+
+            Body = Serializer.Deserialize<TBody>( ms );
+
+            // the rest of the data is the payload
+            int payloadLen = ( int )( ms.Length - ms.Position );
+
+            if ( payloadLen > 0 )
+            {
+                ms.CopyTo( Payload, payloadLen );
+                Payload.Seek( 0, SeekOrigin.Begin );
+            }
         }
 
         /// <summary>
@@ -210,28 +222,6 @@ public override byte[] Serialize()
 
             return ms.ToArray();
         }
-        /// <summary>
-        /// Initializes this client message by deserializing the specified data.
-        /// </summary>
-        /// <param name="data">The data representing a client message.</param>
-        public override void Deserialize( byte[] data )
-        {
-            if ( data == null )
-            {
-                throw new ArgumentNullException( nameof(data) );
-            }
-
-            using MemoryStream ms = new MemoryStream( data );
-            Header.Deserialize( ms );
-            Body = Serializer.Deserialize<TBody>( ms );
-
-            // the rest of the data is the payload
-            int payloadOffset = ( int )ms.Position;
-            int payloadLen = ( int )( ms.Length - ms.Position );
-
-            Payload.Write( data, payloadOffset, payloadLen );
-            Payload.Seek( 0, SeekOrigin.Begin );
-        }
     }
 
     /// <summary>
@@ -357,9 +347,27 @@ public ClientMsg( IPacketMsg msg )
                 throw new ArgumentNullException( nameof(msg) );
             }
 
-            DebugLog.Assert( !msg.IsProto, "ClientMsg", $"ClientMsg<{typeof(TBody).FullName}> used for proto message!" );
+            if ( !( msg is PacketClientMsg packetMsg ) )
+            {
+                throw new InvalidDataException( $"ClientMsg<{typeof( TBody ).FullName}> used for proto message!" );
+            }
+
+            Header = packetMsg.Header;
+
+            var data = packetMsg.GetData();
+            var offset = (int)packetMsg.BodyOffset;
+            using MemoryStream ms = new MemoryStream( data, offset, data.Length - offset );
+
+            Body.Deserialize( ms );
 
-            Deserialize( msg.GetData() );
+            // the rest of the data is the payload
+            int payloadLen = ( int )( ms.Length - ms.Position );
+
+            if ( payloadLen > 0 )
+            {
+                ms.CopyTo( Payload, payloadLen );
+                Payload.Seek( 0, SeekOrigin.Begin );
+            }
         }
 
         /// <summary>
@@ -377,28 +385,6 @@ public override byte[] Serialize()
 
             return ms.ToArray();
         }
-        /// <summary>
-        /// Initializes this client message by deserializing the specified data.
-        /// </summary>
-        /// <param name="data">The data representing a client message.</param>
-        public override void Deserialize( byte[] data )
-        {
-            if ( data == null )
-            {
-                throw new ArgumentNullException( nameof(data) );
-            }
-
-            using MemoryStream ms = new MemoryStream( data );
-            Header.Deserialize( ms );
-            Body.Deserialize( ms );
-
-            // the rest of the data is the payload
-            int payloadOffset = ( int )ms.Position;
-            int payloadLen = ( int )( ms.Length - ms.Position );
-
-            Payload.Write( data, payloadOffset, payloadLen );
-            Payload.Seek( 0, SeekOrigin.Begin );
-        }
     }
 
     /// <summary>
@@ -515,7 +501,27 @@ public Msg( IPacketMsg msg )
                 throw new ArgumentNullException( nameof(msg) );
             }
 
-            Deserialize( msg.GetData() );
+            if ( !( msg is PacketMsg packetMsg ) )
+            {
+                throw new InvalidDataException( $"ClientMsg<{typeof( TBody ).FullName}> used for proto message!" );
+            }
+
+            Header = packetMsg.Header;
+
+            var data = packetMsg.GetData();
+            var offset = (int)packetMsg.BodyOffset;
+            using MemoryStream ms = new MemoryStream( data, offset, data.Length - offset );
+
+            Body.Deserialize( ms );
+
+            // the rest of the data is the payload
+            int payloadLen = ( int )( ms.Length - ms.Position );
+
+            if ( payloadLen > 0 )
+            {
+                ms.CopyTo( Payload, payloadLen );
+                Payload.Seek( 0, SeekOrigin.Begin );
+            }
         }
 
 
@@ -534,28 +540,5 @@ public override byte[] Serialize()
 
             return ms.ToArray();
         }
-        /// <summary>
-        /// Initializes this client message by deserializing the specified data.
-        /// </summary>
-        /// <param name="data">The data representing a client message.</param>
-        public override void Deserialize( byte[] data )
-        {
-            if ( data == null )
-            {
-                throw new ArgumentNullException( nameof(data) );
-            }
-
-            using MemoryStream ms = new MemoryStream( data );
-            Header.Deserialize( ms );
-            Body.Deserialize( ms );
-
-            // the rest of the data is the payload
-            int payloadOffset = ( int )ms.Position;
-            int payloadLen = ( int )( ms.Length - ms.Position );
-
-            Payload.Write( data, payloadOffset, payloadLen );
-            Payload.Seek( 0, SeekOrigin.Begin );
-        }
-
     }
 }
diff --git a/SteamKit2/SteamKit2/Base/MsgBase.cs b/SteamKit2/SteamKit2/Base/MsgBase.cs
index 880812d4e..a3b66c012 100644
--- a/SteamKit2/SteamKit2/Base/MsgBase.cs
+++ b/SteamKit2/SteamKit2/Base/MsgBase.cs
@@ -65,11 +65,6 @@ public interface IClientMsg
         /// </summary>
         /// <returns>Data representing a client message.</returns>
         byte[] Serialize();
-        /// <summary>
-        /// Initializes this client message by deserializing the specified data.
-        /// </summary>
-        /// <param name="data">The data representing a client message.</param>
-        void Deserialize( byte[] data );
     }
 
     /// <summary>
@@ -492,7 +487,7 @@ public abstract class MsgBase<THeader> : MsgBase, IClientMsg
         /// <summary>
         /// Gets the header for this message type. 
         /// </summary>
-        public THeader Header { get; }
+        public THeader Header { get; internal set; }
 
 
         /// <summary>
@@ -513,11 +508,5 @@ public MsgBase( int payloadReserve = 0 )
         /// Data representing a client message.
         /// </returns>
         public abstract byte[] Serialize();
-        /// <summary>
-        /// Initializes this client message by deserializing the specified data.
-        /// </summary>
-        /// <param name="data">The data representing a client message.</param>
-        public abstract void Deserialize( byte[] data );
-
     }
 }
diff --git a/SteamKit2/SteamKit2/Base/PacketBase.cs b/SteamKit2/SteamKit2/Base/PacketBase.cs
index c98030569..a4648d292 100644
--- a/SteamKit2/SteamKit2/Base/PacketBase.cs
+++ b/SteamKit2/SteamKit2/Base/PacketBase.cs
@@ -80,7 +80,7 @@ public sealed class PacketClientMsgProtobuf : IPacketMsg
         /// <value>
         /// 	<c>true</c> if this instance is protobuf backed; otherwise, <c>false</c>.
         /// </value>
-        public bool IsProto { get { return true; } }
+        public bool IsProto => true;
         /// <summary>
         /// Gets the network message type of this packet message.
         /// </summary>
@@ -95,15 +95,28 @@ public sealed class PacketClientMsgProtobuf : IPacketMsg
         /// <value>
         /// The target job id.
         /// </value>
-        public ulong TargetJobID { get; }
+        public ulong TargetJobID => Header.Proto.jobid_target;
         /// <summary>
         /// Gets the source job id for this packet message.
         /// </summary>
         /// <value>
         /// The source job id.
         /// </value>
-        public ulong SourceJobID { get; }
-
+        public ulong SourceJobID => Header.Proto.jobid_source;
+        /// <summary>
+        /// Gets the header for this packet message.
+        /// </summary>
+        /// <value>
+        /// The header.
+        /// </value>
+        internal MsgHdrProtoBuf Header;
+        /// <summary>
+        /// Gets the offset in payload to the body after the header.
+        /// </summary>
+        /// <value>
+        /// The offset in payload after the header.
+        /// </value>
+        internal long BodyOffset;
 
         byte[] payload;
 
@@ -123,16 +136,12 @@ public PacketClientMsgProtobuf( EMsg eMsg, byte[] data )
             MsgType = eMsg;
             payload = data;
 
-            MsgHdrProtoBuf protobufHeader = new MsgHdrProtoBuf();
+            Header = new MsgHdrProtoBuf();
 
             // we need to pull out the job ids, so we deserialize the protobuf header
-            using ( MemoryStream ms = new MemoryStream( data ) )
-            {
-                protobufHeader.Deserialize( ms );
-            }
-
-            TargetJobID = protobufHeader.Proto.jobid_target;
-            SourceJobID = protobufHeader.Proto.jobid_source;
+            using MemoryStream ms = new MemoryStream( data );
+            Header.Deserialize( ms );
+            BodyOffset = ms.Position;
         }
 
 
@@ -173,14 +182,28 @@ public sealed class PacketClientMsg : IPacketMsg
         /// <value>
         /// The target job id.
         /// </value>
-        public ulong TargetJobID { get; }
+        public ulong TargetJobID => Header.TargetJobID;
         /// <summary>
         /// Gets the source job id for this packet message.
         /// </summary>
         /// <value>
         /// The source job id.
         /// </value>
-        public ulong SourceJobID { get; }
+        public ulong SourceJobID => Header.SourceJobID;
+        /// <summary>
+        /// Gets the header for this packet message.
+        /// </summary>
+        /// <value>
+        /// The header.
+        /// </value>
+        internal ExtendedClientMsgHdr Header;
+        /// <summary>
+        /// Gets the offset in payload to the body after the header.
+        /// </summary>
+        /// <value>
+        /// The offset in payload after the header.
+        /// </value>
+        internal long BodyOffset;
 
         byte[] payload;
 
@@ -200,16 +223,12 @@ public PacketClientMsg( EMsg eMsg, byte[] data )
             MsgType = eMsg;
             payload = data;
 
-            ExtendedClientMsgHdr extendedHdr = new ExtendedClientMsgHdr();
+            Header = new ExtendedClientMsgHdr();
 
             // deserialize the extended header to get our hands on the job ids
-            using ( MemoryStream ms = new MemoryStream( data ) )
-            {
-                extendedHdr.Deserialize( ms );
-            }
-
-            TargetJobID = extendedHdr.TargetJobID;
-            SourceJobID = extendedHdr.SourceJobID;
+            using MemoryStream ms = new MemoryStream( data );
+            Header.Deserialize( ms );
+            BodyOffset = ms.Position;
         }
 
 
@@ -250,14 +269,28 @@ public sealed class PacketMsg : IPacketMsg
         /// <value>
         /// The target job id.
         /// </value>
-        public ulong TargetJobID { get; }
+        public ulong TargetJobID => Header.TargetJobID;
         /// <summary>
         /// Gets the source job id for this packet message.
         /// </summary>
         /// <value>
         /// The source job id.
         /// </value>
-        public ulong SourceJobID { get; }
+        public ulong SourceJobID => Header.SourceJobID;
+        /// <summary>
+        /// Gets the header for this packet message.
+        /// </summary>
+        /// <value>
+        /// The header.
+        /// </value>
+        internal MsgHdr Header;
+        /// <summary>
+        /// Gets the offset in payload to the body after the header.
+        /// </summary>
+        /// <value>
+        /// The offset in payload after the header.
+        /// </value>
+        internal long BodyOffset;
 
         byte[] payload;
 
@@ -277,16 +310,12 @@ public PacketMsg( EMsg eMsg, byte[] data )
             MsgType = eMsg;
             payload = data;
 
-            MsgHdr msgHdr = new MsgHdr();
+            Header = new MsgHdr();
 
             // deserialize the header to get our hands on the job ids
-            using ( MemoryStream ms = new MemoryStream( data ) )
-            {
-                msgHdr.Deserialize( ms );
-            }
-
-            TargetJobID = msgHdr.TargetJobID;
-            SourceJobID = msgHdr.SourceJobID;
+            using MemoryStream ms = new MemoryStream( data );
+            Header.Deserialize( ms );
+            BodyOffset = ms.Position;
         }
 
 
diff --git a/SteamKit2/SteamKit2/Steam/Handlers/SteamUnifiedMessages/Callbacks.cs b/SteamKit2/SteamKit2/Steam/Handlers/SteamUnifiedMessages/Callbacks.cs
index a979a1deb..a7884f911 100644
--- a/SteamKit2/SteamKit2/Steam/Handlers/SteamUnifiedMessages/Callbacks.cs
+++ b/SteamKit2/SteamKit2/Steam/Handlers/SteamUnifiedMessages/Callbacks.cs
@@ -4,10 +4,8 @@
  */
 
 using System;
-using System.IO;
 using System.Reflection;
 using ProtoBuf;
-using SteamKit2.Internal;
 
 namespace SteamKit2
 {
@@ -23,17 +21,12 @@ public class ServiceMethodResponse : CallbackMsg
             /// </summary>
             public EResult Result { get; private set; }
 
-            /// <summary>
-            /// Gets the raw binary response.
-            /// </summary>
-            public byte[] ResponseRaw { get; private set; }
-
             /// <summary>
             /// Gets the name of the Service.
             /// </summary>
             public string ServiceName
             {
-                get { return MethodName.Split( '.' )[0]; }
+                get { return MethodName.Split( '.' )[ 0 ]; }
             }
 
             /// <summary>
@@ -41,7 +34,7 @@ public string ServiceName
             /// </summary>
             public string RpcName
             {
-                get { return MethodName.Substring( ServiceName.Length + 1 ).Split( '#' )[0]; }
+                get { return MethodName.Substring( ServiceName.Length + 1 ).Split( '#' )[ 0 ]; }
             }
 
             /// <summary>
@@ -49,14 +42,15 @@ public string RpcName
             /// </summary>
             public string MethodName { get; private set; }
 
+            private PacketClientMsgProtobuf PacketMsg;
 
-            internal ServiceMethodResponse( JobID jobID, EResult result, CMsgClientServiceMethodLegacyResponse resp )
+            internal ServiceMethodResponse( PacketClientMsgProtobuf packetMsg )
             {
-                JobID = jobID;
-
-                Result = result;
-                ResponseRaw = resp.serialized_method_response;
-                MethodName = resp.method_name ?? string.Empty;
+                var protoHeader = packetMsg.Header.Proto;
+                JobID = protoHeader.jobid_target;
+                Result = ( EResult )protoHeader.eresult;
+                MethodName = protoHeader.target_job_name;
+                PacketMsg = packetMsg;
             }
 
 
@@ -66,10 +60,10 @@ internal ServiceMethodResponse( JobID jobID, EResult result, CMsgClientServiceMe
             /// <typeparam name="T">Protobuf type of the response message.</typeparam>
             /// <returns>The response to the message sent through <see cref="SteamUnifiedMessages"/>.</returns>
             public T GetDeserializedResponse<T>()
-                where T : IExtensible
+                where T : IExtensible, new()
             {
-                using var ms = new MemoryStream( ResponseRaw );
-                return Serializer.Deserialize<T>( ms );
+                var msg = new ClientMsgProtobuf<T>( PacketMsg );
+                return msg.Body;
             }
         }
 
@@ -83,7 +77,7 @@ public class ServiceMethodNotification : CallbackMsg
             /// </summary>
             public string ServiceName
             {
-                get { return MethodName.Split( '.' )[0]; }
+                get { return MethodName.Split( '.' )[ 0 ]; }
             }
 
             /// <summary>
@@ -91,7 +85,7 @@ public string ServiceName
             /// </summary>
             public string RpcName
             {
-                get { return MethodName.Substring( ServiceName.Length + 1 ).Split( '#' )[0]; }
+                get { return MethodName.Substring( ServiceName.Length + 1 ).Split( '#' )[ 0 ]; }
             }
 
             /// <summary>
diff --git a/SteamKit2/SteamKit2/Steam/Handlers/SteamUnifiedMessages/SteamUnifiedMessages.cs b/SteamKit2/SteamKit2/Steam/Handlers/SteamUnifiedMessages/SteamUnifiedMessages.cs
index 505df00d3..d2c9b5515 100644
--- a/SteamKit2/SteamKit2/Steam/Handlers/SteamUnifiedMessages/SteamUnifiedMessages.cs
+++ b/SteamKit2/SteamKit2/Steam/Handlers/SteamUnifiedMessages/SteamUnifiedMessages.cs
@@ -10,7 +10,6 @@
 using System.Linq;
 using System.Linq.Expressions;
 using ProtoBuf;
-using SteamKit2.Internal;
 
 namespace SteamKit2
 {
@@ -105,7 +104,7 @@ internal SteamUnifiedMessages()
         {
             dispatchMap = new Dictionary<EMsg, Action<IPacketMsg>>
             {
-                { EMsg.ClientServiceMethodLegacyResponse, HandleClientServiceMethodResponse },
+                { EMsg.ServiceMethodResponse, HandleServiceMethodResponse },
                 { EMsg.ServiceMethod, HandleServiceMethod },
             };
         }
@@ -121,25 +120,17 @@ internal SteamUnifiedMessages()
         /// <param name="isNotification">Whether this message is a notification or not.</param>
         /// <returns>The JobID of the request. This can be used to find the appropriate <see cref="ServiceMethodResponse"/>.</returns>
         public AsyncJob<ServiceMethodResponse> SendMessage<TRequest>( string name, TRequest message, bool isNotification = false )
-            where TRequest : IExtensible
+            where TRequest : IExtensible, new()
         {
             if ( message == null )
             {
                 throw new ArgumentNullException( nameof(message) );
             }
 
-            var msg = new ClientMsgProtobuf<CMsgClientServiceMethodLegacy>( EMsg.ClientServiceMethodLegacy );
+            var msg = new ClientMsgProtobuf<TRequest>( isNotification ? EMsg.ServiceMethodSendToClient : EMsg.ServiceMethodCallFromClient );
             msg.SourceJobID = Client.GetNextJobID();
-
-            using ( var ms = new MemoryStream() )
-            {
-                Serializer.Serialize( ms, message );
-                msg.Body.serialized_method = ms.ToArray();
-            }
-
-            msg.Body.method_name = name;
-            msg.Body.is_notification = isNotification;
-
+            msg.Header.Proto.target_job_name = name;
+            msg.Body = message;
             Client.Send( msg );
 
             return new AsyncJob<ServiceMethodResponse>( this.Client, msg.SourceJobID );
@@ -178,18 +169,25 @@ public override void HandleMsg( IPacketMsg packetMsg )
 
 
         #region ClientMsg Handlers
-        void HandleClientServiceMethodResponse( IPacketMsg packetMsg )
+        void HandleServiceMethodResponse( IPacketMsg packetMsg )
         {
-            var response = new ClientMsgProtobuf<CMsgClientServiceMethodLegacyResponse>( packetMsg );
-            var callback = new ServiceMethodResponse(response.TargetJobID, (EResult)response.ProtoHeader.eresult, response.Body);
+            if ( !( packetMsg is PacketClientMsgProtobuf packetMsgProto ) )
+            {
+                throw new InvalidDataException( "Packet message is expected to be protobuf." );
+            }
+
+            var callback = new ServiceMethodResponse( packetMsgProto );
             Client.PostCallback( callback );
         }
 
         void HandleServiceMethod( IPacketMsg packetMsg )
         {
-            var notification = new ClientMsgProtobuf( packetMsg );
+            if ( !( packetMsg is PacketClientMsgProtobuf packetMsgProto ) )
+            {
+                throw new InvalidDataException( "Packet message is expected to be protobuf." );
+            }
 
-            var jobName = notification.Header.Proto.target_job_name;
+            var jobName = packetMsgProto.Header.Proto.target_job_name;
             if ( !string.IsNullOrEmpty( jobName ) )
             {
                 var splitByDot = jobName.Split( '.' );