JsonRpcMessage.cs

using System.ComponentModel;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace ModelContextProtocol.Protocol;

/// <summary>
/// Represents any JSON-RPC message used in the Model Context Protocol (MCP).
/// </summary>
/// <remarks>
/// This interface serves as the foundation for all message types in the JSON-RPC 2.0 protocol
/// used by MCP, including requests, responses, notifications, and errors. JSON-RPC is a stateless,
/// lightweight remote procedure call (RPC) protocol that uses JSON as its data format.
/// </remarks>
[JsonConverter(typeof(Converter))]
public abstract class JsonRpcMessage
{
    /// <summary>
    /// Gets the JSON-RPC protocol version used.
    /// </summary>
    /// <inheritdoc />
    [JsonPropertyName("jsonrpc")]
    public string JsonRpc { get; init; } = "2.0";

    /// <summary>
    /// Gets or sets the transport the <see cref="JsonRpcMessage"/> was received on or should be sent over.
    /// </summary>
    /// <remarks>
    /// This is used to support the Streamable HTTP transport where the specification states that the server
    /// SHOULD include JSON-RPC responses in the HTTP response body for the POST request containing
    /// the corresponding JSON-RPC request. It may be <see langword="null"/> for other transports.
    /// </remarks>
    [JsonIgnore]
    public ITransport? RelatedTransport { get; set; }

    /// <summary>
    /// Provides a <see cref="JsonConverter"/> for <see cref="JsonRpcMessage"/> messages,
    /// handling polymorphic deserialization of different message types.
    /// </summary>
    /// <remarks>
    /// <para>
    /// This converter is responsible for correctly deserializing JSON-RPC messages into their appropriate
    /// concrete types based on the message structure. It analyzes the JSON payload and determines if it
    /// represents a request, notification, successful response, or error response.
    /// </para>
    /// <para>
    /// The type determination rules follow the JSON-RPC 2.0 specification:
    /// <list type="bullet">
    /// <item><description>Messages with "method" and "id" properties are deserialized as <see cref="JsonRpcRequest"/>.</description></item>
    /// <item><description>Messages with "method" but no "id" property are deserialized as <see cref="JsonRpcNotification"/>.</description></item>
    /// <item><description>Messages with "id" and "result" properties are deserialized as <see cref="JsonRpcResponse"/>.</description></item>
    /// <item><description>Messages with "id" and "error" properties are deserialized as <see cref="JsonRpcError"/>.</description></item>
    /// </list>
    /// </para>
    /// </remarks>
    [EditorBrowsable(EditorBrowsableState.Never)]
    public sealed class Converter : JsonConverter<JsonRpcMessage>
    {
        /// <inheritdoc/>
        public override JsonRpcMessage? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
            if (reader.TokenType != JsonTokenType.StartObject)
            {
                throw new JsonException("Expected StartObject token");
            }

            using var doc = JsonDocument.ParseValue(ref reader);
            var root = doc.RootElement;

            // All JSON-RPC messages must have a jsonrpc property with value "2.0"
            if (!root.TryGetProperty("jsonrpc", out var versionProperty) ||
                versionProperty.GetString() != "2.0")
            {
                throw new JsonException("Invalid or missing jsonrpc version");
            }

            // Determine the message type based on the presence of id, method, and error properties
            bool hasId = root.TryGetProperty("id", out _);
            bool hasMethod = root.TryGetProperty("method", out _);
            bool hasError = root.TryGetProperty("error", out _);

            var rawText = root.GetRawText();

            // Messages with an id but no method are responses
            if (hasId && !hasMethod)
            {
                // Messages with an error property are error responses
                if (hasError)
                {
                    return JsonSerializer.Deserialize(rawText, options.GetTypeInfo<JsonRpcError>());
                }

                // Messages with a result property are success responses
                if (root.TryGetProperty("result", out _))
                {
                    return JsonSerializer.Deserialize(rawText, options.GetTypeInfo<JsonRpcResponse>());
                }

                throw new JsonException("Response must have either result or error");
            }

            // Messages with a method but no id are notifications
            if (hasMethod && !hasId)
            {
                return JsonSerializer.Deserialize(rawText, options.GetTypeInfo<JsonRpcNotification>());
            }

            // Messages with both method and id are requests
            if (hasMethod && hasId)
            {
                return JsonSerializer.Deserialize(rawText, options.GetTypeInfo<JsonRpcRequest>());
            }

            throw new JsonException("Invalid JSON-RPC message format");
        }

        /// <inheritdoc/>
        public override void Write(Utf8JsonWriter writer, JsonRpcMessage value, JsonSerializerOptions options)
        {
            switch (value)
            {
                case JsonRpcRequest request:
                    JsonSerializer.Serialize(writer, request, options.GetTypeInfo<JsonRpcRequest>());
                    break;
                case JsonRpcNotification notification:
                    JsonSerializer.Serialize(writer, notification, options.GetTypeInfo<JsonRpcNotification>());
                    break;
                case JsonRpcResponse response:
                    JsonSerializer.Serialize(writer, response, options.GetTypeInfo<JsonRpcResponse>());
                    break;
                case JsonRpcError error:
                    JsonSerializer.Serialize(writer, error, options.GetTypeInfo<JsonRpcError>());
                    break;
                default:
                    throw new JsonException($"Unknown JSON-RPC message type: {value.GetType()}");
            }
        }
    }
}