diff --git a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIClientExtensions.cs b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIClientExtensions.cs
index d353d46855f..801305ad43a 100644
--- a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIClientExtensions.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIClientExtensions.cs
@@ -187,8 +187,8 @@ internal static BinaryData ToOpenAIFunctionParameters(AIFunctionDeclaration aiFu
             StrictSchemaTransformCache.GetOrCreateTransformedSchema(aiFunction) :
             aiFunction.JsonSchema;
 
-        // Roundtrip the schema through the ToolJson model type to remove extra properties
-        // and force missing ones into existence, then return the serialized UTF8 bytes as BinaryData.
+        // Roundtrip the schema through the ToolJson model type to force missing properties
+        // into existence, then return the serialized UTF8 bytes as BinaryData.
         var tool = JsonSerializer.Deserialize(jsonSchema, OpenAIJsonContext.Default.ToolJson)!;
         var functionParameters = BinaryData.FromBytes(JsonSerializer.SerializeToUtf8Bytes(tool, OpenAIJsonContext.Default.ToolJson));
 
@@ -262,5 +262,8 @@ internal sealed class ToolJson
 
         [JsonPropertyName("additionalProperties")]
         public bool AdditionalProperties { get; set; }
+
+        [JsonExtensionData]
+        public Dictionary<string, JsonElement>? ExtensionData { get; set; }
     }
 }
diff --git a/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIConversionTests.cs b/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIConversionTests.cs
index 1aa7e1e4d0f..7ff028ec736 100644
--- a/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIConversionTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIConversionTests.cs
@@ -104,6 +104,49 @@ public void AsOpenAIChatTool_ProducesValidInstance()
         ValidateSchemaParameters(tool.FunctionParameters);
     }
 
+    [Fact]
+    public void AsOpenAIChatTool_PreservesExtraTopLevelPropertiesLikeDefs()
+    {
+        // Create a JSON schema with $defs (used for reference types)
+        var jsonSchema = JsonDocument.Parse("""
+            {
+                "type": "object",
+                "properties": {
+                    "person": { "$ref": "#/$defs/Person" }
+                },
+                "required": ["person"],
+                "$defs": {
+                    "Person": {
+                        "type": "object",
+                        "properties": {
+                            "name": { "type": "string" }
+                        }
+                    }
+                }
+            }
+            """).RootElement;
+
+        var functionWithDefs = AIFunctionFactory.CreateDeclaration(
+            "test_function_with_defs",
+            "A test function with $defs",
+            jsonSchema);
+
+        var tool = functionWithDefs.AsOpenAIChatTool();
+
+        Assert.NotNull(tool);
+        Assert.Equal("test_function_with_defs", tool.FunctionName);
+        Assert.Equal("A test function with $defs", tool.FunctionDescription);
+
+        // Verify that $defs is preserved in the function parameters
+        using var parsedParams = JsonDocument.Parse(tool.FunctionParameters);
+        var root = parsedParams.RootElement;
+
+        Assert.True(root.TryGetProperty("$defs", out var defs), "The $defs property should be preserved in the function parameters");
+        Assert.True(defs.TryGetProperty("Person", out var person), "The Person definition should exist in $defs");
+        Assert.True(person.TryGetProperty("properties", out var properties), "Person should have properties");
+        Assert.True(properties.TryGetProperty("name", out _), "Person should have a name property");
+    }
+
     [Fact]
     public void AsOpenAIResponseTool_ProducesValidInstance()
     {