.Net: fix: prevent duplicate "null" in JSON Schema type arrays for nullable parameters

dotnet/src/Connectors/Connectors.OpenAI.UnitTests/Core/OpenAIFunctionTests.cs

@@ -299,6 +299,72 @@ public void ItCleansUpRestrictedSchemaKeywords(string typeName, string keyword,
         }
     }
 
+    [Fact]
+    public void ItDoesNotInsertDuplicateNullInTypeArrayForOptionalParameter()
+    {
+        // Arrange — schema with type array already containing "null" (as AIJsonUtilities produces for Nullable<T>)
+        var parameterSchema = KernelJsonSchema.Parse("""{"type":["string","null"],"description":"A nullable param"}""");
+        OpenAIFunction f = KernelFunctionFactory.CreateFromMethod(
+            () => { },
+            parameters: [new KernelParameterMetadata("param1") { Description = "A nullable param", IsRequired = false, Schema = parameterSchema }]).Metadata.ToOpenAIFunction();
+
+        // Act
+        ChatTool result = f.ToFunctionDefinition(strict: true);
+        ParametersData pd = JsonSerializer.Deserialize<ParametersData>(result.FunctionParameters.ToString())!;
+
+        // Assert
+        Assert.NotNull(pd.properties);
+        Assert.Single(pd.properties);
+        var expectedSchema = """{"type":["string","null"],"description":"A nullable param"}""";
+        Assert.Equal(
+            JsonSerializer.Serialize(KernelJsonSchema.Parse(expectedSchema)),
+            JsonSerializer.Serialize(pd.properties.First().Value.RootElement));
+    }
+
+    [Fact]
+    public void ItDoesNotInsertDuplicateNullInTypeArrayForNullableKeyword()
+    {
+        // Arrange — schema with "nullable": true and type array already containing "null"
+        var parameterSchema = KernelJsonSchema.Parse("""{"type":["string","null"],"nullable":true,"description":"A nullable param"}""");
+        OpenAIFunction f = KernelFunctionFactory.CreateFromMethod(
+            () => { },
+            parameters: [new KernelParameterMetadata("param1") { Description = "A nullable param", IsRequired = true, Schema = parameterSchema }]).Metadata.ToOpenAIFunction();
+
+        // Act
+        ChatTool result = f.ToFunctionDefinition(strict: true);
+        ParametersData pd = JsonSerializer.Deserialize<ParametersData>(result.FunctionParameters.ToString())!;
+
+        // Assert — "nullable" keyword is removed in strict mode, type array should not gain duplicate "null"
+        Assert.NotNull(pd.properties);
+        Assert.Single(pd.properties);
+        var expectedSchema = """{"type":["string","null"],"description":"A nullable param"}""";
+        Assert.Equal(
+            JsonSerializer.Serialize(KernelJsonSchema.Parse(expectedSchema)),
+            JsonSerializer.Serialize(pd.properties.First().Value.RootElement));
+    }
+
+    [Fact]
+    public void ItInsertsNullInTypeArrayWhenAbsent()
+    {
+        // Arrange — schema with type array that does NOT contain "null"
+        var parameterSchema = KernelJsonSchema.Parse("""{"type":["string"],"description":"An optional param"}""");
+        OpenAIFunction f = KernelFunctionFactory.CreateFromMethod(
+            () => { },
+            parameters: [new KernelParameterMetadata("param1") { Description = "An optional param", IsRequired = false, Schema = parameterSchema }]).Metadata.ToOpenAIFunction();
+
+        // Act
+        ChatTool result = f.ToFunctionDefinition(strict: true);
+        ParametersData pd = JsonSerializer.Deserialize<ParametersData>(result.FunctionParameters.ToString())!;
+
+        // Assert — "null" should be added to the type array
+        Assert.NotNull(pd.properties);
+        Assert.Single(pd.properties);
+        var expectedSchema = """{"type":["string","null"],"description":"An optional param"}""";
+        Assert.Equal(
+            JsonSerializer.Serialize(KernelJsonSchema.Parse(expectedSchema)),
+            JsonSerializer.Serialize(pd.properties.First().Value.RootElement));
+    }
+
 #pragma warning disable CA1812 // uninstantiated internal class
     private sealed class ParametersData
     {

dotnet/src/Connectors/Connectors.OpenAI/Core/OpenAIFunction.cs

@@ -313,7 +313,8 @@ private static void InsertNullTypeIfRequired(bool insertNullType, JsonObject jso
         {
             return;
         }
-        if (typeValue is JsonArray jsonArray && !jsonArray.Contains(NullType))
+        if (typeValue is JsonArray jsonArray &&
+            !jsonArray.Any(static x => x is JsonValue jv && jv.GetValueKind() == JsonValueKind.String && jv.GetValue<string>() == NullType))
         {
             jsonArray.Add(NullType);
         }