diff --git a/Anthropic.SDK/Anthropic.SDK.csproj b/Anthropic.SDK/Anthropic.SDK.csproj
index bc886d6..cd49dff 100644
--- a/Anthropic.SDK/Anthropic.SDK.csproj
+++ b/Anthropic.SDK/Anthropic.SDK.csproj
@@ -14,12 +14,12 @@
 	  <PackageTags>Claude, AI, ML, API, Anthropic</PackageTags>
 	  <Title>Claude API</Title>
 	  <PackageReleaseNotes>
-		  Updates Microsoft.Extensions.AI.Abstractions, Bug Fixes
+		  Sonnet 4.5, Caching Enhancements, Minor Improvements
 	  </PackageReleaseNotes>
 	  <PackageId>Anthropic.SDK</PackageId>
-	  <Version>5.5.2</Version>
+	  <Version>5.5.3</Version>
 	  <AssemblyVersion>5.5.0.0</AssemblyVersion>
-	  <FileVersion>5.5.2.0</FileVersion>
+	  <FileVersion>5.5.3.0</FileVersion>
 	  <GenerateDocumentationFile>True</GenerateDocumentationFile>
 	  <PackageReadmeFile>README.md</PackageReadmeFile>
 	  <PackageIcon>icon.png</PackageIcon>
diff --git a/Anthropic.SDK/Constants/AnthropicModels.cs b/Anthropic.SDK/Constants/AnthropicModels.cs
index 87bfcac..5ef4b62 100644
--- a/Anthropic.SDK/Constants/AnthropicModels.cs
+++ b/Anthropic.SDK/Constants/AnthropicModels.cs
@@ -30,6 +30,11 @@ public static class AnthropicModels
         /// </summary>
         public const string Claude4Sonnet = "claude-sonnet-4-20250514";
 
+        /// <summary>
+        /// Claude 4.5 Sonnet
+        /// </summary>
+        public const string Claude45Sonnet = "claude-sonnet-4-5-20250929";
+
         /// <summary>
         /// Claude 4 Opus
         /// </summary>
diff --git a/Anthropic.SDK/Constants/VertexAIModels.cs b/Anthropic.SDK/Constants/VertexAIModels.cs
index fbf244a..6438516 100644
--- a/Anthropic.SDK/Constants/VertexAIModels.cs
+++ b/Anthropic.SDK/Constants/VertexAIModels.cs
@@ -35,10 +35,15 @@ public static class VertexAIModels
         /// </summary>
         public const string Claude4Sonnet = "claude-sonnet-4@20250514";
 
+        /// <summary>
+        /// Claude 4.5 Sonnet
+        /// </summary>
+        public const string Claude45Sonnet = "claude-sonnet-4@20250514";
+
         /// <summary>
         /// Claude 4 Opus
         /// </summary>
-        public const string Claude4Opus = "claude-opus-4@20250514";
+        public const string Claude4Opus = "claude-sonnet-4-5@20250929";
 
         /// <summary>
         /// Claude 4.1 Opus
diff --git a/README.md b/README.md
index d550434..2ffb9c4 100644
--- a/README.md
+++ b/README.md
@@ -1207,7 +1207,8 @@ Vertex AI provides access to the following Claude models:
 - `VertexAIModels.Claude35Sonnet`: High level of intelligence and capability
 - `VertexAIModels.Claude35Haiku`: Intelligence at blazing speeds
 - `VertexAIModels.Claude37Sonnet`: Highest level of intelligence and capability with toggleable extended thinking
-- `VertexAIModels.Claude4Sonnet`: Newest Sonnet model with toggleable extended thinking
+- `VertexAIModels.Claude4Sonnet`: Sonnet model with toggleable extended thinking
+- `VertexAIModels.Claude45Sonnet`: Newest Sonnet model with toggleable extended thinking
 - `VertexAIModels.Claude4Opus`: Previous Opus Model and powerful thinking model
 - `VertexAIModels.Claude41Opus`: Newest Opus Model and most powerful thinking model