.Net InputVariable.Default type is changed from string to object?

dotnet/src/Connectors/Connectors.OpenAI/AzureSdk/OpenAIKernelFunctionMetadataExtensions.cs

@@ -22,9 +22,10 @@ public static OpenAIFunction ToOpenAIFunction(this KernelFunctionMetadata metada
         for (int i = 0; i < openAIParams.Length; i++)
         {
             var param = metadataParams[i];
+
             openAIParams[i] = new OpenAIFunctionParameter(
                 param.Name,
-                string.IsNullOrEmpty(param.DefaultValue) ? param.Description : $"{param.Description} (default value: {param.DefaultValue})",
+                GetDescription(param),
                 param.IsRequired,
                 param.ParameterType,
                 param.Schema);
@@ -39,5 +40,15 @@ public static OpenAIFunction ToOpenAIFunction(this KernelFunctionMetadata metada
                 metadata.ReturnParameter.Description,
                 metadata.ReturnParameter.ParameterType,
                 metadata.ReturnParameter.Schema));
+
+        static string GetDescription(KernelParameterMetadata param)
+        {
+            if (InternalTypeConverter.ConvertToString(param.DefaultValue) is string stringValue && !string.IsNullOrEmpty(stringValue))
+            {
+                return $"{param.Description} (default value: {stringValue})";
+            }
+
+            return param.Description;
+        }
     }
 }

dotnet/src/Extensions/PromptTemplates.Handlebars/HandlebarsPromptTemplate.cs

@@ -98,10 +98,12 @@ private KernelArguments GetVariables(KernelArguments? arguments)
 
         foreach (var p in this._promptModel.InputVariables)
         {
-            if (!string.IsNullOrEmpty(p.Default))
+            if (p.Default == null || (p.Default is string stringDefault && stringDefault.Length == 0))
             {
-                result[p.Name] = p.Default;
+                continue;
             }
+
+            result[p.Name] = p.Default;
         }
 
         if (arguments is not null)

dotnet/src/Functions/Functions.UnitTests/Yaml/Functions/KernelFunctionYamlTests.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System;
 using Microsoft.SemanticKernel;
 using Microsoft.SemanticKernel.Connectors.OpenAI;
 using Xunit;
@@ -10,6 +11,15 @@ namespace SemanticKernel.Functions.UnitTests.Yaml;
 
 public class KernelFunctionYamlTests
 {
+    private readonly ISerializer _serializer;
+
+    public KernelFunctionYamlTests()
+    {
+        this._serializer = new SerializerBuilder()
+            .WithNamingConvention(UnderscoredNamingConvention.Instance)
+            .Build();
+    }
+
     [Fact]
     public void ItShouldCreateFunctionFromPromptYamlWithNoExecutionSettings()
     {
@@ -72,78 +82,118 @@ public void ItShouldSupportCreatingOpenAIExecutionSettings()
         Assert.Equal(0.0, executionSettings.TopP);
     }
 
+    [Fact]
+    public void ItShouldCreateFunctionWithDefaultValueOfStringType()
+    {
+        // Act
+        var function = KernelFunctionYaml.FromPromptYaml(this._yamlWithCustomSettings);
+
+        // Assert
+        Assert.NotNull(function?.Metadata?.Parameters);
+        Assert.Equal("John", function?.Metadata?.Parameters[0].DefaultValue);
+        Assert.Equal("English", function?.Metadata?.Parameters[1].DefaultValue);
+    }
+
+    [Fact]
+    // This test checks that the logic of imposing a temporary limitation on the default value being a string is in place and works as expected.
+    public void ItShouldThrowExceptionWhileCreatingFunctionWithDefaultValueOtherThanString()
+    {
+        string CreateYaml(object defaultValue)
+        {
+            var obj = new
+            {
+                description = "function description",
+                input_variables = new[]
+                {
+                    new
+                    {
+                        name = "name",
+                        description = "description",
+                        @default = defaultValue,
+                        isRequired = true
+                    }
+                }
+            };
+
+            return this._serializer.Serialize(obj);
+        }
+
+        // Act
+        Assert.Throws<NotSupportedException>(() => KernelFunctionYaml.FromPromptYaml(CreateYaml(new { p1 = "v1" })));
+    }
+
     private readonly string _yamlNoExecutionSettings = @"
-    template_format: semantic-kernel
-    template:        Say hello world to {{$name}} in {{$language}}
-    description:     Say hello to the specified person using the specified language
-    name:            SayHello
-    input_variables:
-      - name:          name
-        description:   The name of the person to greet
-        default:       John
-      - name:          language
-        description:   The language to generate the greeting in
-        default: English
-";
+        template_format: semantic-kernel
+        template:        Say hello world to {{$name}} in {{$language}}
+        description:     Say hello to the specified person using the specified language
+        name:            SayHello
+        input_variables:
+          - name:          name
+            description:   The name of the person to greet
+            default:       John
+          - name:          language
+            description:   The language to generate the greeting in
+            default: English
+        ";
 
     private readonly string _yaml = @"
-    template_format: semantic-kernel
-    template:        Say hello world to {{$name}} in {{$language}}
-    description:     Say hello to the specified person using the specified language
-    name:            SayHello
-    input_variables:
-      - name:          name
-        description:   The name of the person to greet
-        default:       John
-      - name:          language
-        description:   The language to generate the greeting in
-        default: English
-    execution_settings:
-      service1:
-        model_id:          gpt-4
-        temperature:       1.0
-        top_p:             0.0
-        presence_penalty:  0.0
-        frequency_penalty: 0.0
-        max_tokens:        256
-        stop_sequences:    []
-      service2:
-        model_id:          gpt-3.5
-        temperature:       1.0
-        top_p:             0.0
-        presence_penalty:  0.0
-        frequency_penalty: 0.0
-        max_tokens:        256
-        stop_sequences:    [ ""foo"", ""bar"", ""baz"" ]
-";
+        template_format: semantic-kernel
+        template:        Say hello world to {{$name}} in {{$language}}
+        description:     Say hello to the specified person using the specified language
+        name:            SayHello
+        input_variables:
+          - name:          name
+            description:   The name of the person to greet
+            default:       John
+          - name:          language
+            description:   The language to generate the greeting in
+            default: English
+        execution_settings:
+          service1:
+            model_id:          gpt-4
+            temperature:       1.0
+            top_p:             0.0
+            presence_penalty:  0.0
+            frequency_penalty: 0.0
+            max_tokens:        256
+            stop_sequences:    []
+          service2:
+            model_id:          gpt-3.5
+            temperature:       1.0
+            top_p:             0.0
+            presence_penalty:  0.0
+            frequency_penalty: 0.0
+            max_tokens:        256
+            stop_sequences:    [ ""foo"", ""bar"", ""baz"" ]
+        ";
 
     private readonly string _yamlWithCustomSettings = @"
-    template_format: semantic-kernel
-    template:        Say hello world to {{$name}} in {{$language}}
-    description:     Say hello to the specified person using the specified language
-    name:            SayHello
-    input_variables:
-      - name:          name
-        description:   The name of the person to greet
-        default:       John
-      - name:          language
-        description:   The language to generate the greeting in
-        default: English
-    execution_settings:
-      service1:
-        model_id:          gpt-4
-        temperature:       1.0
-        top_p:             0.0
-        presence_penalty:  0.0
-        frequency_penalty: 0.0
-        max_tokens:        256
-        stop_sequences:    []
-      service2:
-        model_id:          random-model
-        temperaturex:      1.0
-        top_q:             0.0
-        rando_penalty:     0.0
-        max_token_count:   256
-        stop_sequences:    [ ""foo"", ""bar"", ""baz"" ]
-";
+        template_format: semantic-kernel
+        template:        Say hello world to {{$name}} in {{$language}}
+        description:     Say hello to the specified person using the specified language
+        name:            SayHello
+        input_variables:
+          - name:          name
+            description:   The name of the person to greet
+            default:       John
+          - name:          language
+            description:   The language to generate the greeting in
+            default:       English
+        execution_settings:
+          service1:
+            model_id:          gpt-4
+            temperature:       1.0
+            top_p:             0.0
+            presence_penalty:  0.0
+            frequency_penalty: 0.0
+            max_tokens:        256
+            stop_sequences:    []
+          service2:
+            model_id:          random-model
+            temperaturex:      1.0
+            top_q:             0.0
+            rando_penalty:     0.0
+            max_token_count:   256
+            stop_sequences:    [ ""foo"", ""bar"", ""baz"" ]
+        ";
 }

dotnet/src/Functions/Functions.Yaml/KernelFunctionYaml.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+using System;
 using Microsoft.Extensions.Logging;
 using YamlDotNet.Serialization;
 using YamlDotNet.Serialization.NamingConventions;
@@ -33,6 +34,20 @@ public static KernelFunction FromPromptYaml(
 
         var promptTemplateConfig = deserializer.Deserialize<PromptTemplateConfig>(text);
 
+        // Prevent the default value from being any type other than a string.
+        // It's a temporary limitation that helps shape the public API surface
+        // (changing the type of the Default property to object) now, before the release.
+        // This helps avoid a breaking change while a proper solution for
+        // dealing with the different deserialization outputs of JSON/YAML prompt configurations is being evaluated.
+        foreach (var inputVariable in promptTemplateConfig.InputVariables)
+        {
+            if (inputVariable.Default is not null && inputVariable.Default is not string)
+            {
+                throw new NotSupportedException($"Default value for input variable '{inputVariable.Name}' must be a string. " +
+                        $"This is a temporary limitation; future updates are expected to remove this constraint. Prompt function - '{promptTemplateConfig.Name ?? promptTemplateConfig.Description}'.");
+            }
+        }
+
         return KernelFunctionFactory.CreateFromPrompt(
             promptTemplateConfig,
             promptTemplateFactory,

dotnet/src/InternalUtilities/planning/Extensions/KernelFunctionMetadataExtensions.cs

@@ -66,16 +66,14 @@ internal static string ToManualString(this KernelFunctionMetadata function)
     {
         var inputs = string.Join("\n", function.Parameters.Select(parameter =>
         {
-            var defaultValueString = string.IsNullOrEmpty(parameter.DefaultValue) ? string.Empty : $" (default value: {parameter.DefaultValue})";
+            var defaultValueString = InternalTypeConverter.ConvertToString(parameter.DefaultValue);
+            defaultValueString = string.IsNullOrEmpty(defaultValueString) ? string.Empty : $" (default value: {defaultValueString})";
             return $"    - {parameter.Name}: {parameter.Description}{defaultValueString}";
         }));
 
         // description and inputs are indented by 2 spaces
         // While each parameter in inputs is indented by 4 spaces
-        return $@"{function.ToFullyQualifiedName()}:
-  description: {function.Description}
-  inputs:
-{inputs}";
+        return $"{function.ToFullyQualifiedName()}:  description: {function.Description}  inputs:{inputs}";
     }
 
     /// <summary>

dotnet/src/InternalUtilities/src/System/InternalTypeConverter.cs

@@ -18,7 +18,7 @@ internal static class InternalTypeConverter
     /// <param name="value">The object to convert.</param>
     /// <param name="culture">The CultureInfo to consider during conversion.</param>
     /// <returns>A string representation of the object value, considering the specified CultureInfo.</returns>
-    public static string? ConvertToString(object? value, CultureInfo culture)
+    public static string? ConvertToString(object? value, CultureInfo? culture = null)
     {
         if (value == null) { return null; }
 
@@ -28,7 +28,7 @@ internal static class InternalTypeConverter
 
         return converterDelegate == null
             ? value.ToString()
-            : converterDelegate(value, culture);
+            : converterDelegate(value, culture ?? CultureInfo.InvariantCulture);
     }
 
     /// <summary>

dotnet/src/SemanticKernel.Abstractions/Functions/KernelParameterMetadata.cs

@@ -18,7 +18,7 @@ public sealed class KernelParameterMetadata
     /// <summary>The description of the parameter.</summary>
     private string _description = string.Empty;
     /// <summary>The default value of the parameter.</summary>
-    private string? _defaultValue;
+    private object? _defaultValue;
     /// <summary>The .NET type of the parameter.</summary>
     private Type? _parameterType;
     /// <summary>The schema of the parameter, potentially lazily-initialized.</summary>
@@ -72,7 +72,7 @@ public string Description
     }
 
     /// <summary>Gets the default value of the parameter.</summary>
-    public string? DefaultValue
+    public object? DefaultValue
     {
         get => this._defaultValue;
         init
@@ -113,7 +113,7 @@ public KernelJsonSchema? Schema
     /// <param name="parameterType">The parameter type. If null, no schema can be inferred.</param>
     /// <param name="defaultValue">The parameter's default value, if any.</param>
     /// <param name="description">The parameter description. If null, it won't be included in the schema.</param>
-    internal static InitializedSchema InferSchema(Type? parameterType, string? defaultValue, string? description)
+    internal static InitializedSchema InferSchema(Type? parameterType, object? defaultValue, string? description)
     {
         KernelJsonSchema? schema = null;
 
@@ -134,10 +134,10 @@ internal static InitializedSchema InferSchema(Type? parameterType, string? defau
             {
                 try
                 {
-                    if (!string.IsNullOrWhiteSpace(defaultValue))
+                    if (InternalTypeConverter.ConvertToString(defaultValue) is string stringDefault && !string.IsNullOrWhiteSpace(stringDefault))
                     {
                         bool needsSpace = !string.IsNullOrWhiteSpace(description);
-                        description += $"{(needsSpace ? " " : "")}(default value: {defaultValue})";
+                        description += $"{(needsSpace ? " " : "")}(default value: {stringDefault})";
                     }
 
                     var builder = new JsonSchemaBuilder().FromType(parameterType);

dotnet/src/SemanticKernel.Abstractions/PromptTemplate/InputVariable.cs

@@ -47,7 +47,7 @@ public string Description
     /// Gets or sets a default value for the variable.
     /// </summary>
     [JsonPropertyName("default")]
-    public string? Default { get; set; }
+    public object? Default { get; set; }
 
     /// <summary>
     /// Gets or sets whether the variable is considered required (rather than optional).