diff --git a/src/AnthropicClient/AnthropicApiClient.cs b/src/AnthropicClient/AnthropicApiClient.cs
index ad5d0c8..2c57cdd 100644
--- a/src/AnthropicClient/AnthropicApiClient.cs
+++ b/src/AnthropicClient/AnthropicApiClient.cs
@@ -191,6 +191,8 @@ public class AnthropicApiClient : IAnthropicApiClient
         {
           InputTokens = existingUsage.InputTokens + msgDeltaData.Usage.InputTokens,
           OutputTokens = existingUsage.OutputTokens + msgDeltaData.Usage.OutputTokens,
+          CacheCreationInputTokens = existingUsage.CacheCreationInputTokens + msgDeltaData.Usage.CacheCreationInputTokens,
+          CacheReadInputTokens = existingUsage.CacheReadInputTokens + msgDeltaData.Usage.CacheReadInputTokens,
         };
 
         msgResponse = new MessageResponse()
diff --git a/src/AnthropicClient/Json/JsonSerializationOptions.cs b/src/AnthropicClient/Json/JsonSerializationOptions.cs
index a5a9ce8..6e9dece 100644
--- a/src/AnthropicClient/Json/JsonSerializationOptions.cs
+++ b/src/AnthropicClient/Json/JsonSerializationOptions.cs
@@ -17,7 +17,6 @@ static class JsonSerializationOptions
       new EventDataConverter(),
       new ContentDeltaConverter(),
       new JsonStringEnumConverter(),
-      new MessageRequestConverter(),
     },
     DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
   };
diff --git a/src/AnthropicClient/Models/BaseMessageRequest.cs b/src/AnthropicClient/Models/BaseMessageRequest.cs
index 7765310..fb5413b 100644
--- a/src/AnthropicClient/Models/BaseMessageRequest.cs
+++ b/src/AnthropicClient/Models/BaseMessageRequest.cs
@@ -15,15 +15,40 @@ public abstract class BaseMessageRequest
   public string Model { get; init; } = string.Empty;
 
   /// <summary>
-  /// Gets the system prompt to use for the request.
+  /// Gets the system message that will be used as the system prompt if no system messages are provided.
   /// </summary>
+  [JsonIgnore]
   public string? System { get; init; } = null;
 
   /// <summary>
-  /// Gets the messages to send to the model.
+  /// Gets the system messages to send to the model to be used as the system prompt.
   /// </summary>
+  [JsonIgnore]
   public List<TextContent>? SystemMessages { get; init; } = null;
 
+  /// <summary>
+  /// Gets the system prompt that will be used for the request. 
+  /// If will return the system messages if they are provided, otherwise it will return the system message.
+  /// If neither are provided, it will return null.
+  /// </summary>
+  [JsonPropertyName("system")]
+  public List<TextContent>? SystemPrompt => GetSystemPrompt();
+
+  private List<TextContent>? GetSystemPrompt()
+  {
+    if (SystemMessages is not null)
+    {
+      return SystemMessages;
+    }
+
+    if (System is not null)
+    {
+      return [new TextContent(System)];
+    }
+
+    return null;
+  }
+
   /// <summary>
   /// Gets the messages to send to the model.
   /// </summary>
diff --git a/tests/AnthropicClient.Tests/Unit/Models/AnthropicEventTests.cs b/tests/AnthropicClient.Tests/Unit/Models/AnthropicEventTests.cs
index 931616b..28da978 100644
--- a/tests/AnthropicClient.Tests/Unit/Models/AnthropicEventTests.cs
+++ b/tests/AnthropicClient.Tests/Unit/Models/AnthropicEventTests.cs
@@ -13,7 +13,9 @@ public class AnthropicEventTests : SerializationTest
         ""stop_sequence"": """",
         ""usage"": {
           ""input_tokens"": 472,
-          ""output_tokens"": 2
+          ""output_tokens"": 2,
+          ""cache_creation_input_tokens"": 0,
+          ""cache_read_input_tokens"": 0
         },
         ""content"": [],
         ""stop_reason"": """"
@@ -70,7 +72,9 @@ public class AnthropicEventTests : SerializationTest
       },
       ""usage"": {
         ""output_tokens"": 89,
-        ""input_tokens"": 0
+        ""input_tokens"": 0,
+        ""cache_creation_input_tokens"": 0,
+        ""cache_read_input_tokens"": 0
       },
       ""type"": ""message_delta""
     }
diff --git a/tests/AnthropicClient.Tests/Unit/Models/MessageDeltaEventDataTests.cs b/tests/AnthropicClient.Tests/Unit/Models/MessageDeltaEventDataTests.cs
index 0d22412..b164915 100644
--- a/tests/AnthropicClient.Tests/Unit/Models/MessageDeltaEventDataTests.cs
+++ b/tests/AnthropicClient.Tests/Unit/Models/MessageDeltaEventDataTests.cs
@@ -10,7 +10,9 @@ public class MessageDeltaEventDataTests : SerializationTest
     },
     ""usage"": {
       ""input_tokens"": 1,
-      ""output_tokens"": 1
+      ""output_tokens"": 1, 
+      ""cache_creation_input_tokens"": 1,
+      ""cache_read_input_tokens"": 1
     }
   }";
 
@@ -18,7 +20,13 @@ public class MessageDeltaEventDataTests : SerializationTest
   public void Constructor_WhenCalled_ItShouldInitializeProperties()
   {
     var expectedDelta = new MessageDelta("max_tokens", "max_tokens");
-    var expectedUsage = new Usage { InputTokens = 1, OutputTokens = 1 };
+    var expectedUsage = new Usage 
+    { 
+      InputTokens = 1, 
+      OutputTokens = 1,
+      CacheCreationInputTokens = 1,
+      CacheReadInputTokens = 1,
+    };
 
     var messageDeltaEventData = new MessageDeltaEventData(expectedDelta, expectedUsage);
 
@@ -30,7 +38,13 @@ public class MessageDeltaEventDataTests : SerializationTest
   public void JsonSerialization_WhenSerialized_ItShouldHaveExpectedShape()
   {
     var expectedDelta = new MessageDelta("max_tokens", "max_tokens");
-    var expectedUsage = new Usage { InputTokens = 1, OutputTokens = 1 };
+    var expectedUsage = new Usage 
+    { 
+      InputTokens = 1, 
+      OutputTokens = 1,
+      CacheCreationInputTokens = 1,
+      CacheReadInputTokens = 1,
+    };
 
     var messageDeltaEventData = new MessageDeltaEventData(expectedDelta, expectedUsage);
 
@@ -43,7 +57,13 @@ public class MessageDeltaEventDataTests : SerializationTest
   public void JsonDeserialization_WhenDeserialized_ItShouldHaveExpectedValues()
   {
     var expectedDelta = new MessageDelta("max_tokens", "max_tokens");
-    var expectedUsage = new Usage { InputTokens = 1, OutputTokens = 1 };
+    var expectedUsage = new Usage 
+    { 
+      InputTokens = 1, 
+      OutputTokens = 1,
+      CacheCreationInputTokens = 1,
+      CacheReadInputTokens = 1,
+    };
 
     var messageDeltaEventData = Deserialize<MessageDeltaEventData>(_testJson);
 
diff --git a/tests/AnthropicClient.Tests/Unit/Models/MessageRequestTests.cs b/tests/AnthropicClient.Tests/Unit/Models/MessageRequestTests.cs
index a1ae3b0..bcd3532 100644
--- a/tests/AnthropicClient.Tests/Unit/Models/MessageRequestTests.cs
+++ b/tests/AnthropicClient.Tests/Unit/Models/MessageRequestTests.cs
@@ -4,7 +4,10 @@ public class MessageRequestTests : SerializationTest
 {
   private readonly string _testJson = @"{
     ""model"": ""claude-3-sonnet-20240229"",
-    ""system"": ""test-system"",
+    ""system"": [{
+      ""type"": ""text"",
+      ""text"": ""test-system""
+    }],
     ""messages"": [
       { ""role"": ""user"", ""content"": [{ ""text"": ""Hello!"", ""type"": ""text"" }] }
     ],
@@ -21,7 +24,10 @@ public class MessageRequestTests : SerializationTest
 
   private readonly string _testJsonWithAnyToolChoice = @"{
     ""model"": ""claude-3-sonnet-20240229"",
-    ""system"": ""test-system"", 
+    ""system"": [{
+      ""type"": ""text"",
+      ""text"": ""test-system""
+    }], 
     ""messages"":[
       { ""role"": ""user"", ""content"": [{ ""text"": ""Hello!"", ""type"":""text"" }] }
     ],
@@ -38,7 +44,10 @@ public class MessageRequestTests : SerializationTest
 
   private readonly string _testJsonWithSpecificToolChoice = @"{
     ""model"": ""claude-3-sonnet-20240229"",
-    ""system"": ""test-system"",
+    ""system"": [{
+      ""type"": ""text"",
+      ""text"": ""test-system""
+    }],
     ""messages"": [
       { ""role"": ""user"", ""content"": [{ ""text"": ""Hello!"", ""type"": ""text"" }] }
     ],
@@ -55,7 +64,10 @@ public class MessageRequestTests : SerializationTest
 
   private readonly string _testJsonWithImageContent = @"{
     ""model"": ""claude-3-sonnet-20240229"",
-    ""system"": ""test-system"",
+    ""system"": [{
+      ""type"": ""text"",
+      ""text"": ""test-system""
+    }],
     ""messages"":[
       { 
         ""role"": ""user"", 
@@ -80,7 +92,10 @@ public class MessageRequestTests : SerializationTest
 
   private readonly string _testJsonWithUnknownContent = @"{
     ""model"": ""claude-3-sonnet-20240229"",
-    ""system"": ""test-system"",
+    ""system"": [{
+      ""type"": ""text"",
+      ""text"": ""test-system""
+    }],
     ""messages"": [{ ""role"": ""user"", ""content"": [{ ""type"": ""unknown"", ""text"": ""text"" }] }],
     ""max_tokens"": 512,
     ""metadata"": { ""test"": ""test"" },
@@ -95,7 +110,10 @@ public class MessageRequestTests : SerializationTest
 
   private readonly string _testJsonWithToolUseContent = @"{
     ""model"": ""claude-3-sonnet-20240229"",
-    ""system"": ""test-system"",
+    ""system"": [{
+      ""type"": ""text"",
+      ""text"": ""test-system""
+    }],
     ""messages"": [
       {
         ""role"": ""assistant"",
@@ -124,7 +142,10 @@ public class MessageRequestTests : SerializationTest
 
   private readonly string _testJsonWithToolResultContent = @"{
     ""model"": ""claude-3-sonnet-20240229"",
-    ""system"": ""test-system"",
+    ""system"": [{
+      ""type"": ""text"",
+      ""text"": ""test-system""
+    }],
     ""messages"": [
       {
         ""role"": ""assistant"",
@@ -307,7 +328,7 @@ public class MessageRequestTests : SerializationTest
     var messageRequest = Deserialize<MessageRequest>(_testJson);
 
     messageRequest!.Model.Should().Be(AnthropicModels.Claude3Sonnet);
-    messageRequest.System.Should().Be("test-system");
+    messageRequest.System.Should().BeNull();
     messageRequest.Messages.Should().HaveCount(1);
     messageRequest.MaxTokens.Should().Be(512);
     messageRequest.Metadata.Should().HaveCount(1);
@@ -330,7 +351,7 @@ public class MessageRequestTests : SerializationTest
     var messageRequest = Deserialize<MessageRequest>(_testJsonWithAnyToolChoice);
 
     messageRequest!.Model.Should().Be(AnthropicModels.Claude3Sonnet);
-    messageRequest.System.Should().Be("test-system");
+    messageRequest.System.Should().BeNull();
     messageRequest.Messages.Should().HaveCount(1);
     messageRequest.MaxTokens.Should().Be(512);
     messageRequest.Metadata.Should().HaveCount(1);
@@ -352,7 +373,7 @@ public class MessageRequestTests : SerializationTest
     var messageRequest = Deserialize<MessageRequest>(_testJsonWithSpecificToolChoice);
 
     messageRequest!.Model.Should().Be(AnthropicModels.Claude3Sonnet);
-    messageRequest.System.Should().Be("test-system");
+    messageRequest.System.Should().BeNull();
     messageRequest.Messages.Should().HaveCount(1);
     messageRequest.MaxTokens.Should().Be(512);
     messageRequest.Metadata.Should().HaveCount(1);
@@ -387,7 +408,7 @@ public class MessageRequestTests : SerializationTest
     var messageRequest = Deserialize<MessageRequest>(_testJsonWithImageContent);
 
     messageRequest!.Model.Should().Be(AnthropicModels.Claude3Sonnet);
-    messageRequest.System.Should().Be("test-system");
+    messageRequest.System.Should().BeNull();
     messageRequest.Messages.Should().HaveCount(1);
     messageRequest.MaxTokens.Should().Be(512);
     messageRequest.Metadata.Should().HaveCount(1);
@@ -416,7 +437,7 @@ public class MessageRequestTests : SerializationTest
     var messageRequest = Deserialize<MessageRequest>(_testJsonWithToolUseContent);
 
     messageRequest!.Model.Should().Be(AnthropicModels.Claude3Sonnet);
-    messageRequest.System.Should().Be("test-system");
+    messageRequest.System.Should().BeNull();
     messageRequest.Messages.Should().HaveCount(1);
     messageRequest.MaxTokens.Should().Be(512);
     messageRequest.Metadata.Should().HaveCount(1);
@@ -447,7 +468,7 @@ public class MessageRequestTests : SerializationTest
     var messageRequest = Deserialize<MessageRequest>(_testJsonWithToolResultContent);
 
     messageRequest!.Model.Should().Be(AnthropicModels.Claude3Sonnet);
-    messageRequest.System.Should().Be("test-system");
+    messageRequest.System.Should().BeNull();
     messageRequest.Messages.Should().HaveCount(1);
     messageRequest.MaxTokens.Should().Be(512);
     messageRequest.Metadata.Should().HaveCount(1);
diff --git a/tests/AnthropicClient.Tests/Unit/Models/MessageResponseTests.cs b/tests/AnthropicClient.Tests/Unit/Models/MessageResponseTests.cs
index e2bf053..43a3e02 100644
--- a/tests/AnthropicClient.Tests/Unit/Models/MessageResponseTests.cs
+++ b/tests/AnthropicClient.Tests/Unit/Models/MessageResponseTests.cs
@@ -53,7 +53,12 @@ public class MessageResponseTests : SerializationTest
       ""stop_reason"": ""stop reason"",
       ""stop_sequence"": ""stop sequence"",
       ""type"": ""type"",
-      ""usage"": { ""input_tokens"": 1, ""output_tokens"": 2 },
+      ""usage"": { 
+        ""input_tokens"": 1, 
+        ""output_tokens"": 2,
+        ""cache_creation_input_tokens"": 0,
+        ""cache_read_input_tokens"": 0
+      },
       ""content"": [
         { ""text"": ""text content"", ""type"": ""text"" }
       ]
@@ -93,7 +98,12 @@ public class MessageResponseTests : SerializationTest
       ""stop_reason"": ""stop reason"",
       ""stop_sequence"": ""stop sequence"",
       ""type"": ""type"",
-      ""usage"": { ""input_tokens"": 1, ""output_tokens"": 2 },
+      ""usage"": { 
+        ""input_tokens"": 1, 
+        ""output_tokens"": 2,
+        ""cache_creation_input_tokens"": 0,
+        ""cache_read_input_tokens"": 0
+      },
       ""content"": [
         { ""text"": ""text content"", ""type"": ""text"" }
       ]
diff --git a/tests/AnthropicClient.Tests/Unit/Models/MessageStartEventDataTests.cs b/tests/AnthropicClient.Tests/Unit/Models/MessageStartEventDataTests.cs
index 3a550d8..bc6a5fd 100644
--- a/tests/AnthropicClient.Tests/Unit/Models/MessageStartEventDataTests.cs
+++ b/tests/AnthropicClient.Tests/Unit/Models/MessageStartEventDataTests.cs
@@ -14,7 +14,9 @@ public class MessageStartEventDataTests : SerializationTest
       ""stop_sequence"": """",
       ""usage"": {
         ""input_tokens"": 25,
-        ""output_tokens"": 1
+        ""output_tokens"": 1,
+        ""cache_creation_input_tokens"": 0,
+        ""cache_read_input_tokens"": 0
       }
     }
   }";
diff --git a/tests/AnthropicClient.Tests/Unit/Models/StreamMessageRequestTests.cs b/tests/AnthropicClient.Tests/Unit/Models/StreamMessageRequestTests.cs
index 16c1274..67ebd1e 100644
--- a/tests/AnthropicClient.Tests/Unit/Models/StreamMessageRequestTests.cs
+++ b/tests/AnthropicClient.Tests/Unit/Models/StreamMessageRequestTests.cs
@@ -4,7 +4,10 @@ public class StreamMessageRequestTests : SerializationTest
 {
   private readonly string _testJson = @"{
     ""model"": ""claude-3-sonnet-20240229"",
-    ""system"": ""test-system"",
+    ""system"": [{
+      ""type"": ""text"",
+      ""text"": ""test-system""
+    }],
     ""messages"": [
       { ""role"": ""user"", ""content"": [{ ""text"": ""Hello!"", ""type"": ""text"" }] }
     ],
@@ -178,7 +181,7 @@ public class StreamMessageRequestTests : SerializationTest
     var messageRequest = Deserialize<StreamMessageRequest>(_testJson);
 
     messageRequest!.Model.Should().Be(AnthropicModels.Claude3Sonnet);
-    messageRequest.System.Should().Be("test-system");
+    messageRequest.System.Should().BeNull();
     messageRequest.Messages.Should().HaveCount(1);
     messageRequest.MaxTokens.Should().Be(512);
     messageRequest.Metadata.Should().HaveCount(1);
diff --git a/tests/AnthropicClient.Tests/Unit/Models/UsageTests.cs b/tests/AnthropicClient.Tests/Unit/Models/UsageTests.cs
index 345527a..a0ca68f 100644
--- a/tests/AnthropicClient.Tests/Unit/Models/UsageTests.cs
+++ b/tests/AnthropicClient.Tests/Unit/Models/UsageTests.cs
@@ -7,26 +7,39 @@ public class UsageTests : SerializationTest
   {
     var expectedInputTokens = 1;
     var expectedOutputTokens = 2;
+    var expectedCacheCreationInputTokens = 3;
+    var expectedCacheReadInputTokens = 4;
 
     var usage = new Usage
     {
       InputTokens = expectedInputTokens,
-      OutputTokens = expectedOutputTokens
+      OutputTokens = expectedOutputTokens,
+      CacheCreationInputTokens = expectedCacheCreationInputTokens,
+      CacheReadInputTokens = expectedCacheReadInputTokens
     };
 
     usage.InputTokens.Should().Be(expectedInputTokens);
     usage.OutputTokens.Should().Be(expectedOutputTokens);
+    usage.CacheCreationInputTokens.Should().Be(expectedCacheCreationInputTokens);
+    usage.CacheReadInputTokens.Should().Be(expectedCacheReadInputTokens);
   }
 
   [Fact]
   public void JsonSerialization_WhenCalled_ItShouldSerializeCorrectly()
   {
-    var expectedJson = @"{ ""input_tokens"": 1, ""output_tokens"": 2 }";
+    var expectedJson = @"{ 
+      ""input_tokens"": 1, 
+      ""output_tokens"": 2,
+      ""cache_creation_input_tokens"": 3,
+      ""cache_read_input_tokens"": 4
+    }";
 
     var usage = new Usage
     {
       InputTokens = 1,
-      OutputTokens = 2
+      OutputTokens = 2,
+      CacheCreationInputTokens = 3,
+      CacheReadInputTokens = 4
     };
 
     var actual = Serialize(usage);
@@ -37,11 +50,18 @@ public class UsageTests : SerializationTest
   [Fact]
   public void JsonDeserialization_WhenCalled_ItShouldDeserializeCorrectly()
   {
-    var json = @"{ ""input_tokens"": 1, ""output_tokens"": 2 }";
+    var json = @"{ 
+      ""input_tokens"": 1, 
+      ""output_tokens"": 2,
+      ""cache_creation_input_tokens"": 3,
+      ""cache_read_input_tokens"": 4
+    }";
 
     var usage = Deserialize<Usage>(json);
 
     usage!.InputTokens.Should().Be(1);
     usage.OutputTokens.Should().Be(2);
+    usage.CacheCreationInputTokens.Should().Be(3);
+    usage.CacheReadInputTokens.Should().Be(4);
   }
 }
\ No newline at end of file