diff --git a/OpenAI-DotNet/Chat/ChatRequest.cs b/OpenAI-DotNet/Chat/ChatRequest.cs
index 8e0922d7..3c595e69 100644
--- a/OpenAI-DotNet/Chat/ChatRequest.cs
+++ b/OpenAI-DotNet/Chat/ChatRequest.cs
@@ -267,7 +267,7 @@ public ChatRequest(
         /// </remarks>
         [JsonPropertyName("reasoning_effort")]
         [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
-        public ReasoningEffort ReasoningEffort { get; }
+        public ReasoningEffort? ReasoningEffort { get; }
 
         /// <summary>
         /// Developer-defined tags and values used for filtering completions in the dashboard.
diff --git a/OpenAI-DotNet/OpenAI-DotNet.csproj b/OpenAI-DotNet/OpenAI-DotNet.csproj
index 6e990630..722cb633 100644
--- a/OpenAI-DotNet/OpenAI-DotNet.csproj
+++ b/OpenAI-DotNet/OpenAI-DotNet.csproj
@@ -29,8 +29,10 @@ More context [on Roger Pincombe's blog](https://rogerpincombe.com/openai-dotnet-
     <AssemblyOriginatorKeyFile>OpenAI-DotNet.pfx</AssemblyOriginatorKeyFile>
     <IncludeSymbols>true</IncludeSymbols>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
-    <Version>8.5.0</Version>
+    <Version>8.5.1</Version>
     <PackageReleaseNotes>
+Version 8.5.1
+- Fix ChatRequest serialization for ReasoningEffort
 Version 8.5.0
 - Added Role.Developer for reasoning model messages
 - Added reasoning effort parameter for o1 series models in ChatRequest