Avoid new JsonSerializerOptions instances (#34)

We try hard to avoid creating unnecessary JsonSerializerOptions, as each can have a non-trivial overhead due to how reflection is used by JsonSerializer and the JSO instance serving as a cache.  Rather than creating a new JSO for each transport / endpoint, this can just use a singleton. The only thing it loses is logging when the converter fails, but all of those paths are also throwing an exception, which can be logged higher in the call stack.

src/mcpdotnet/Utils/Json/JsonRpcMessageConverter.cs

@@ -9,21 +9,13 @@ namespace McpDotNet.Utils.Json;
 /// <summary>
 /// JSON converter for IJsonRpcMessage that handles polymorphic deserialization of different message types.
 /// </summary>
-internal class JsonRpcMessageConverter : JsonConverter<IJsonRpcMessage>
+internal sealed class JsonRpcMessageConverter : JsonConverter<IJsonRpcMessage>
 {
-    private readonly ILogger<JsonRpcMessageConverter> _logger;
-
-    public JsonRpcMessageConverter(ILogger<JsonRpcMessageConverter> logger)
-    {
-        _logger = logger;
-    }
-
     /// <inheritdoc/>
     public override IJsonRpcMessage? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
     {
         if (reader.TokenType != JsonTokenType.StartObject)
         {
-            _logger.JsonRpcMessageConverterExpectedStartObjectToken();
             throw new JsonException("Expected StartObject token");
         }
 
@@ -34,7 +26,6 @@ internal class JsonRpcMessageConverter : JsonConverter<IJsonRpcMessage>
         if (!root.TryGetProperty("jsonrpc", out var versionProperty) ||
             versionProperty.GetString() != "2.0")
         {
-            _logger.JsonRpcMessageConverterInvalidJsonRpcVersion();
             throw new JsonException("Invalid or missing jsonrpc version");
         }
 
@@ -51,32 +42,27 @@ internal class JsonRpcMessageConverter : JsonConverter<IJsonRpcMessage>
             // Messages with an error property are error responses
             if (hasError)
             {
-                _logger.JsonRpcMessageConverterDeserializingErrorResponse(rawText);
                 return JsonSerializer.Deserialize<JsonRpcError>(rawText, options);
             }
             // Messages with a result property are success responses
             else if (root.TryGetProperty("result", out _))
             {
-                _logger.JsonRpcMessageConverterDeserializingResponse(rawText);
                 return JsonSerializer.Deserialize<JsonRpcResponse>(rawText, options);
             }
-            _logger.JsonRpcMessageConverterResponseMustHaveResultOrError(rawText);
+
             throw new JsonException("Response must have either result or error");
         }
         // Messages with a method but no id are notifications
         else if (hasMethod && !hasId)
         {
-            _logger.JsonRpcMessageConverterDeserializingNotification(rawText);
             return JsonSerializer.Deserialize<JsonRpcNotification>(rawText, options);
         }
         // Messages with both method and id are requests
         else if (hasMethod && hasId)
         {
-            _logger.JsonRpcMessageConverterDeserializingRequest(rawText);
             return JsonSerializer.Deserialize<JsonRpcRequest>(rawText, options);
         }
 
-        _logger.JsonRpcMessageConverterInvalidMessageFormat(rawText);
         throw new JsonException("Invalid JSON-RPC message format");
     }
 
@@ -98,7 +84,6 @@ internal class JsonRpcMessageConverter : JsonConverter<IJsonRpcMessage>
                 JsonSerializer.Serialize(writer, error, options);
                 break;
             default:
-                _logger.JsonRpcMessageConverterWriteUnknownMessageType(value.GetType().ToString());
                 throw new JsonException($"Unknown JSON-RPC message type: {value.GetType()}");
         }
     }

src/mcpdotnet/Utils/Json/JsonSerializerOptionsExtensions.cs

@@ -3,21 +3,24 @@ using System.Text.Json;
 using System.Text.Json.Serialization;
 
 namespace McpDotNet.Utils.Json;
+
 /// <summary>
 /// Extensions for configuring System.Text.Json serialization options for MCP.
 /// </summary>
 internal static class JsonSerializerOptionsExtensions
 {
+    public static JsonSerializerOptions DefaultOptions { get; } = CreateDefaultOptions();
+
     /// <summary>
-    /// Configures JsonSerializerOptions with MCP-specific settings and converters.
+    /// Creates default options to use for MCP-related serialization.
     /// </summary>
-    /// <param name="options">The options to configure.</param>
-    /// <param name="loggerFactory">The logger factory to use for logging.</param>
     /// <returns>The configured options.</returns>
-    public static JsonSerializerOptions ConfigureForMcp(this JsonSerializerOptions options, ILoggerFactory loggerFactory)
+    private static JsonSerializerOptions CreateDefaultOptions()
     {
+        JsonSerializerOptions options = new();
+
         // Add custom converters
-        options.Converters.Add(new JsonRpcMessageConverter(loggerFactory.CreateLogger<JsonRpcMessageConverter>()));
+        options.Converters.Add(new JsonRpcMessageConverter());
         options.Converters.Add(new JsonStringEnumConverter());
 
         // Configure general options