Fix ToJsonObject serialization failure with anonymous types and add support for custom JsonSerializerOptions

src/ModelContextProtocol.Core/AIContentExtensions.cs

@@ -1,6 +1,7 @@
 using Microsoft.Extensions.AI;
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol;
+using System.Diagnostics.CodeAnalysis;
 #if !NET
 using System.Runtime.InteropServices;
 #endif
@@ -138,8 +139,10 @@ public static class AIContentExtensions
     }
 
     /// <summary>Converts the specified dictionary to a <see cref="JsonObject"/>.</summary>
+    [UnconditionalSuppressMessage("Trimming", "IL2026:Members annotated with 'RequiresUnreferencedCodeAttribute' require dynamic access", Justification = "DefaultOptions includes fallback to reflection-based serialization when available.")]
+    [UnconditionalSuppressMessage("ReflectionAnalysis", "IL3050:RequiresDynamicCode", Justification = "DefaultOptions includes fallback to reflection-based serialization when available.")]
     internal static JsonObject? ToJsonObject(this IReadOnlyDictionary<string, object?> properties) =>
-        JsonSerializer.SerializeToNode(properties, McpJsonUtilities.JsonContext.Default.IReadOnlyDictionaryStringObject) as JsonObject;
+        JsonSerializer.SerializeToNode(properties, typeof(IReadOnlyDictionary<string, object?>), McpJsonUtilities.DefaultOptions) as JsonObject;
 
     internal static AdditionalPropertiesDictionary ToAdditionalProperties(this JsonObject obj)
     {
@@ -271,7 +274,7 @@ public static IList<PromptMessage> ToPromptMessages(this ChatMessage chatMessage
             EmbeddedResourceBlock resourceContent => resourceContent.Resource.ToAIContent(),
 
             ToolUseContentBlock toolUse => FunctionCallContent.CreateFromParsedArguments(toolUse.Input, toolUse.Id, toolUse.Name,
-                static json => JsonSerializer.Deserialize(json, McpJsonUtilities.JsonContext.Default.IDictionaryStringObject)),
+                static json => JsonSerializer.Deserialize(json, McpJsonUtilities.DefaultOptions.GetTypeInfo<IDictionary<string, object?>>())),
 
             ToolResultContentBlock toolResult => new FunctionResultContent(
                 toolResult.ToolUseId,
@@ -367,6 +370,8 @@ public static IList<AIContent> ToAIContents(this IEnumerable<ResourceContents> c
     /// <param name="content">The <see cref="AIContent"/> to convert.</param>
     /// <returns>The created <see cref="ContentBlock"/>.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="content"/> is <see langword="null"/>.</exception>
+    [UnconditionalSuppressMessage("Trimming", "IL2026:Members annotated with 'RequiresUnreferencedCodeAttribute' require dynamic access", Justification = "DefaultOptions includes fallback to reflection-based serialization when available.")]
+    [UnconditionalSuppressMessage("ReflectionAnalysis", "IL3050:RequiresDynamicCode", Justification = "DefaultOptions includes fallback to reflection-based serialization when available.")]
     public static ContentBlock ToContentBlock(this AIContent content)
     {
         Throw.IfNull(content);
@@ -414,13 +419,13 @@ public static ContentBlock ToContentBlock(this AIContent content)
                 Content =
                     resultContent.Result is AIContent c ? [c.ToContentBlock()] :
                     resultContent.Result is IEnumerable<AIContent> ec ? [.. ec.Select(c => c.ToContentBlock())] :
-                    [new TextContentBlock { Text = JsonSerializer.Serialize(content, McpJsonUtilities.DefaultOptions.GetTypeInfo<object>()) }],
+                    [new TextContentBlock { Text = JsonSerializer.Serialize(resultContent.Result, resultContent.Result?.GetType() ?? typeof(object), McpJsonUtilities.DefaultOptions) }],
                 StructuredContent = resultContent.Result is JsonElement je ? je : null,
             },
 
             _ => new TextContentBlock
             {
-                Text = JsonSerializer.Serialize(content, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(object))),
+                Text = $"[Unsupported AIContent type: {content.GetType().Name}]",
             }
         };
 

src/ModelContextProtocol.Core/McpJsonUtilities.cs

@@ -34,8 +34,8 @@ public static partial class McpJsonUtilities
     /// Creates default options to use for MCP-related serialization.
     /// </summary>
     /// <returns>The configured options.</returns>
-    [UnconditionalSuppressMessage("ReflectionAnalysis", "IL3050:RequiresDynamicCode", Justification = "Converter is guarded by IsReflectionEnabledByDefault check.")]
-    [UnconditionalSuppressMessage("Trimming", "IL2026:Members annotated with 'RequiresUnreferencedCodeAttribute' require dynamic access", Justification = "Converter is guarded by IsReflectionEnabledByDefault check.")]
+    [UnconditionalSuppressMessage("ReflectionAnalysis", "IL3050:RequiresDynamicCode", Justification = "Fallback resolver is added only when reflection is enabled or when processing user-defined types that may require reflection.")]
+    [UnconditionalSuppressMessage("Trimming", "IL2026:Members annotated with 'RequiresUnreferencedCodeAttribute' require dynamic access", Justification = "Fallback resolver is added only when reflection is enabled or when processing user-defined types that may require reflection.")]
     private static JsonSerializerOptions CreateDefaultOptions()
     {
         // Copy the configuration from the source generated context.
@@ -44,6 +44,10 @@ private static JsonSerializerOptions CreateDefaultOptions()
         // Chain with all supported types from MEAI.
         options.TypeInfoResolverChain.Add(AIJsonUtilities.DefaultOptions.TypeInfoResolver!);
 
+        // Add a fallback reflection-based resolver for types not covered by source generators.
+        // This allows serialization of user-defined types, including anonymous types in AdditionalProperties.
+        options.TypeInfoResolverChain.Add(new DefaultJsonTypeInfoResolver());
+
         // Add a converter for user-defined enums, if reflection is enabled by default.
         if (JsonSerializer.IsReflectionEnabledByDefault)
         {

tests/ModelContextProtocol.Tests/AIContentExtensionsAnonymousTypeTests.cs

@@ -0,0 +1,135 @@
+using Microsoft.Extensions.AI;
+using ModelContextProtocol.Protocol;
+
+namespace ModelContextProtocol.Tests;
+
+/// <summary>
+/// Tests for AIContentExtensions with anonymous types in AdditionalProperties.
+/// This validates the fix for the sampling pipeline regression in 0.5.0-preview.1.
+/// </summary>
+public class AIContentExtensionsAnonymousTypeTests
+{
+    [Fact]
+    public void ToContentBlock_WithAnonymousTypeInAdditionalProperties_DoesNotThrow()
+    {
+        // This is the minimal repro from the issue
+        AIContent c = new()
+        {
+            AdditionalProperties = new()
+            {
+                ["data"] = new { X = 1.0, Y = 2.0 }
+            }
+        };
+
+        // Should not throw NotSupportedException
+        var contentBlock = c.ToContentBlock();
+
+        Assert.NotNull(contentBlock);
+        Assert.NotNull(contentBlock.Meta);
+        Assert.True(contentBlock.Meta.ContainsKey("data"));
+    }
+
+    [Fact]
+    public void ToContentBlock_WithMultipleAnonymousTypes_DoesNotThrow()
+    {
+        AIContent c = new()
+        {
+            AdditionalProperties = new()
+            {
+                ["point"] = new { X = 1.0, Y = 2.0 },
+                ["metadata"] = new { Name = "Test", Id = 42 },
+                ["config"] = new { Enabled = true, Timeout = 30 }
+            }
+        };
+
+        var contentBlock = c.ToContentBlock();
+
+        Assert.NotNull(contentBlock);
+        Assert.NotNull(contentBlock.Meta);
+        Assert.Equal(3, contentBlock.Meta.Count);
+    }
+
+    [Fact]
+    public void ToContentBlock_WithNestedAnonymousTypes_DoesNotThrow()
+    {
+        AIContent c = new()
+        {
+            AdditionalProperties = new()
+            {
+                ["outer"] = new 
+                { 
+                    Inner = new { Value = "test" },
+                    Count = 5
+                }
+            }
+        };
+
+        var contentBlock = c.ToContentBlock();
+
+        Assert.NotNull(contentBlock);
+        Assert.NotNull(contentBlock.Meta);
+        Assert.True(contentBlock.Meta.ContainsKey("outer"));
+    }
+
+    [Fact]
+    public void ToContentBlock_WithMixedTypesInAdditionalProperties_DoesNotThrow()
+    {
+        AIContent c = new()
+        {
+            AdditionalProperties = new()
+            {
+                ["anonymous"] = new { X = 1.0, Y = 2.0 },
+                ["string"] = "test",
+                ["number"] = 42,
+                ["boolean"] = true,
+                ["array"] = new[] { 1, 2, 3 }
+            }
+        };
+
+        var contentBlock = c.ToContentBlock();
+
+        Assert.NotNull(contentBlock);
+        Assert.NotNull(contentBlock.Meta);
+        Assert.Equal(5, contentBlock.Meta.Count);
+    }
+
+    [Fact]
+    public void TextContent_ToContentBlock_WithAnonymousTypeInAdditionalProperties_PreservesData()
+    {
+        TextContent textContent = new("Hello, world!")
+        {
+            AdditionalProperties = new()
+            {
+                ["location"] = new { Lat = 40.7128, Lon = -74.0060 }
+            }
+        };
+
+        var contentBlock = textContent.ToContentBlock();
+        var textBlock = Assert.IsType<TextContentBlock>(contentBlock);
+
+        Assert.Equal("Hello, world!", textBlock.Text);
+        Assert.NotNull(textBlock.Meta);
+        Assert.True(textBlock.Meta.ContainsKey("location"));
+    }
+
+    [Fact]
+    public void DataContent_ToContentBlock_WithAnonymousTypeInAdditionalProperties_PreservesData()
+    {
+        byte[] imageData = [1, 2, 3, 4, 5];
+        DataContent dataContent = new(imageData, "image/png")
+        {
+            AdditionalProperties = new()
+            {
+                ["dimensions"] = new { Width = 100, Height = 200 }
+            }
+        };
+
+        var contentBlock = dataContent.ToContentBlock();
+        var imageBlock = Assert.IsType<ImageContentBlock>(contentBlock);
+
+        Assert.Equal(Convert.ToBase64String(imageData), imageBlock.Data);
+        Assert.Equal("image/png", imageBlock.MimeType);
+        Assert.NotNull(imageBlock.Meta);
+        Assert.True(imageBlock.Meta.ContainsKey("dimensions"));
+    }
+}

tests/ModelContextProtocol.Tests/McpJsonUtilitiesTests.cs

@@ -18,12 +18,15 @@ public static void DefaultOptions_IsSingleton()
     }
 
     [Fact]
-    public static void DefaultOptions_UseReflectionWhenEnabled()
+    public static void DefaultOptions_SupportsAnonymousTypes()
     {
+        // DefaultOptions includes a fallback DefaultJsonTypeInfoResolver to support
+        // serialization of user-defined types including anonymous types, regardless
+        // of the IsReflectionEnabledByDefault setting.
         var options = McpJsonUtilities.DefaultOptions;
         Type anonType = new { Id = 42 }.GetType();
 
-        Assert.Equal(JsonSerializer.IsReflectionEnabledByDefault, options.TryGetTypeInfo(anonType, out _));
+        Assert.True(options.TryGetTypeInfo(anonType, out _));
     }
 
     [Fact]

tests/ModelContextProtocol.Tests/RegressionTests.cs

@@ -0,0 +1,31 @@
+using Microsoft.Extensions.AI;
+
+namespace ModelContextProtocol.Tests;
+
+/// <summary>
+/// Regression tests for specific issues that were reported and fixed.
+/// </summary>
+public class RegressionTests
+{
+    /// <summary>
+    /// Regression test for GitHub issue: ToJsonObject fails when dictionary values contain anonymous types.
+    /// This is a sampling pipeline regression from version 0.5.0-preview.1.
+    /// </summary>
+    [Fact]
+    public void Issue_AnonymousTypes_InAdditionalProperties_ShouldNotThrow()
+    {
+        // Exact minimal repro from the issue
+        AIContent c = new()
+        {
+            AdditionalProperties = new()
+            {
+                ["data"] = new { X = 1.0, Y = 2.0 }
+            }
+        };
+
+        // This should not throw NotSupportedException
+        var exception = Record.Exception(() => c.ToContentBlock());
+
+        Assert.Null(exception);
+    }
+}