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

src/ModelContextProtocol.Core/AIContentExtensions.cs

@@ -139,7 +139,7 @@ public static class AIContentExtensions
 
     /// <summary>Converts the specified dictionary to a <see cref="JsonObject"/>.</summary>
     internal static JsonObject? ToJsonObject(this IReadOnlyDictionary<string, object?> properties) =>
-        JsonSerializer.SerializeToNode(properties, McpJsonUtilities.JsonContext.Default.IReadOnlyDictionaryStringObject) as JsonObject;
+        JsonSerializer.SerializeToNode(properties, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(IReadOnlyDictionary<string, object?>))) as JsonObject;
 
     internal static AdditionalPropertiesDictionary ToAdditionalProperties(this JsonObject obj)
     {
@@ -271,7 +271,7 @@ 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,

tests/ModelContextProtocol.Tests/AIContentExtensionsTests.cs

@@ -148,4 +148,161 @@ public void ToAIContent_ToolResultToFunctionResultRoundTrip()
         Assert.False(functionResult.Exception != null);
         Assert.NotNull(functionResult.Result);
     }
+
+    // Tests for anonymous types in AdditionalProperties (sampling pipeline regression fix)
+    // These tests require reflection-based serialization and will be skipped when reflection is disabled.
+
+    [Fact]
+    public void ToContentBlock_WithAnonymousTypeInAdditionalProperties_DoesNotThrow()
+    {
+        if (!JsonSerializer.IsReflectionEnabledByDefault)
+        {
+            return;
+        }
+
+        // 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()
+    {
+        if (!JsonSerializer.IsReflectionEnabledByDefault)
+        {
+            return;
+        }
+
+        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()
+    {
+        if (!JsonSerializer.IsReflectionEnabledByDefault)
+        {
+            return;
+        }
+
+        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()
+    {
+        if (!JsonSerializer.IsReflectionEnabledByDefault)
+        {
+            return;
+        }
+
+        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()
+    {
+        if (!JsonSerializer.IsReflectionEnabledByDefault)
+        {
+            return;
+        }
+
+        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()
+    {
+        if (!JsonSerializer.IsReflectionEnabledByDefault)
+        {
+            return;
+        }
+
+        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"));
+    }
 }