Client library reviewed.

code/MessengerApi.Client/Model/Contact.cs

@@ -0,0 +1,9 @@
+namespace MessengerApi.Model
+{
+    public class Contact
+    {
+        public Guid Id { get; set; }
+
+        public string Name { get; set; }
+    }
+}

code/MessengerApi.Client/Model/Credentials.cs

@@ -0,0 +1,15 @@
+namespace MessengerApi.Model
+{
+    public class Credentials
+    {
+        public string ApiKey { get; private set; }
+
+        public string ApiUrl { get; private set; }
+
+        public Credentials(string apiKey, string apiUrl)
+        {
+            ApiKey = apiKey;
+            ApiUrl = apiUrl;
+        }
+    }
+}

code/MessengerApi.Client/Model/Messages/InboxMessage.cs

@@ -0,0 +1,16 @@
+namespace MessengerApi.Model.Messages
+{
+    /// <summary>
+    /// Message when received is inbox. For server apps, this is request-type of message. For clients, this is a response-type of message.
+    /// </summary>
+    public class InboxMessage
+    {
+        public Guid Id { get; set; }
+
+        public Guid Sender { get; set; }
+
+        public string PayloadType { get; set; }
+
+        public string Payload { get; set; }
+    }
+}

code/MessengerApi.Client/Model/Messages/OutboxMessage.cs

@@ -0,0 +1,16 @@
+namespace MessengerApi.Model.Messages
+{
+    /// <summary>
+    /// Outbox type of message. A server-app will treat this as a response. A client app will treat this as a request.
+    /// </summary>
+    public class OutboxMessage
+    {
+        public Guid? ToUserId { get; set; }
+
+        public string PayloadType { get; set; }
+
+        public string Payload { get; set; }
+
+        public int? LifespanInSeconds { get; set; }
+    }
+}

code/MessengerApi.Client/Model/State.cs

@@ -0,0 +1,13 @@
+using System.Text.Json.Serialization;
+
+namespace MessengerApi.Model
+{
+    public class State
+    {
+        [JsonPropertyName("isDelivered")]
+        public bool IsDelivered { get; set; }
+
+        [JsonPropertyName("isAcknowledged")]
+        public bool IsAcknowledged { get; set; }
+    }
+}