Hardcode the source generator in one remaining location and simplify McpJsonUtilities.

src/ModelContextProtocol/Protocol/Transport/SseResponseStreamTransport.cs

@@ -53,7 +53,7 @@ private void WriteJsonRpcMessageToBuffer(SseItem<IJsonRpcMessage?> item, IBuffer
             return;
         }
 
-        JsonSerializer.Serialize(GetUtf8JsonWriter(writer), item.Data, McpJsonUtilities.DefaultOptions.GetTypeInfo<IJsonRpcMessage?>());
+        JsonSerializer.Serialize(GetUtf8JsonWriter(writer), item.Data, McpJsonUtilities.JsonContext.Default.IJsonRpcMessage);
     }
 
     /// <inheritdoc/>

src/ModelContextProtocol/Utils/Json/McpJsonUtilities.cs

@@ -3,7 +3,6 @@
 using ModelContextProtocol.Protocol.Types;
 using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
-using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
 using System.Text.Json.Serialization.Metadata;
 
@@ -35,36 +34,12 @@ public static partial class McpJsonUtilities
     /// Creates default options to use for MCP-related serialization.
     /// </summary>
     /// <returns>The configured options.</returns>
-    [UnconditionalSuppressMessage("AotAnalysis", "IL3050", Justification = "DefaultJsonTypeInfoResolver is only used when reflection-based serialization is enabled")]
-    [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2026", Justification = "DefaultJsonTypeInfoResolver is only used when reflection-based serialization is enabled")]
     private static JsonSerializerOptions CreateDefaultOptions()
     {
-        // If reflection-based serialization is enabled by default, use it, as it's the most permissive in terms of what it can serialize,
-        // and we want to be flexible in terms of what can be put into the various collections in the object model.
-        // Otherwise, use the source-generated options to enable trimming and Native AOT.
-        JsonSerializerOptions options;
+        // Copy the configuration from the source generated context.
+        JsonSerializerOptions options = new(JsonContext.Default.Options);
 
-        if (JsonSerializer.IsReflectionEnabledByDefault)
-        {
-            // Keep in sync with the JsonSourceGenerationOptions attribute on JsonContext below.
-            options = new(JsonSerializerDefaults.Web)
-            {
-                TypeInfoResolver = new DefaultJsonTypeInfoResolver(),
-                Converters = { new JsonStringEnumConverter() },
-                DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
-                NumberHandling = JsonNumberHandling.AllowReadingFromString,
-            };
-        }
-        else
-        {
-            // Keep in sync with any additional settings above beyond what's in JsonContext below.
-            options = new(JsonContext.Default.Options)
-            {
-            };
-        }
-
-        // Include all types from AIJsonUtilities, so that anything default usable as part of an AIFunction
-        // is also usable as part of an McpServerTool.
+        // Chain with all supported types from MEAI
         options.TypeInfoResolverChain.Add(AIJsonUtilities.DefaultOptions.TypeInfoResolver!);
 
         options.MakeReadOnly();
@@ -106,11 +81,6 @@ internal static bool IsValidMcpToolSchema(JsonElement element)
         UseStringEnumConverter = true,
         DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
         NumberHandling = JsonNumberHandling.AllowReadingFromString)]
-
-    // JSON
-    [JsonSerializable(typeof(JsonDocument))]
-    [JsonSerializable(typeof(JsonElement))]
-    [JsonSerializable(typeof(JsonNode))]
 
     // JSON-RPC
     [JsonSerializable(typeof(IJsonRpcMessage))]

tests/ModelContextProtocol.Tests/McpJsonUtilitiesTests.cs

@@ -0,0 +1,28 @@
+using ModelContextProtocol.Utils.Json;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Tests;
+
+public static class McpJsonUtilitiesTests
+{
+    [Fact]
+    public static void DefaultOptions_IsSingleton()
+    {
+        var options = McpJsonUtilities.DefaultOptions;
+
+        Assert.NotNull(options);
+        Assert.True(options.IsReadOnly);
+        Assert.Same(options, McpJsonUtilities.DefaultOptions);
+    }
+
+    [Fact]
+    public static void DefaultOptions_UseReflectionWhenEnabled()
+    {
+        var options = McpJsonUtilities.DefaultOptions;
+        bool isReflectionEnabled = JsonSerializer.IsReflectionEnabledByDefault;
+        Type anonType = new { Id = 42 }.GetType();
+
+        Assert.True(isReflectionEnabled); // To be disabled once https://github.com/dotnet/extensions/pull/6241 is incorporated
+        Assert.Equal(isReflectionEnabled, options.TryGetTypeInfo(anonType, out _));
+    }
+}