Default draft protocol support: sessionless + handshake-less (SEP-2575 + SEP-2567) (#1610)

src/ModelContextProtocol.Core/UnsupportedProtocolVersionException.cs

@@ -0,0 +1,74 @@
+using ModelContextProtocol.Protocol;
+using System.Diagnostics.CodeAnalysis;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace ModelContextProtocol;
+
+/// <summary>
+/// Represents an exception used to signal that a request's declared protocol version is not supported by the server.
+/// </summary>
+/// <remarks>
+/// Introduced by the draft protocol revision (SEP-2575). Servers throw this exception when they cannot process
+/// a request because the per-request <c>_meta/io.modelcontextprotocol/protocolVersion</c> (or the equivalent
+/// transport-level header) names a version the server does not implement. The exception is converted to a
+/// JSON-RPC error response with code <see cref="McpErrorCode.UnsupportedProtocolVersion"/> (<c>-32004</c>) and
+/// a <see cref="UnsupportedProtocolVersionErrorData"/> payload.
+/// </remarks>
+public sealed class UnsupportedProtocolVersionException : McpProtocolException
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="UnsupportedProtocolVersionException"/> class.
+    /// </summary>
+    /// <param name="requested">The protocol version the client requested.</param>
+    /// <param name="supported">The protocol versions the server supports.</param>
+    /// <param name="message">A human-readable description of the error. If <see langword="null"/>, a default message is used.</param>
+    public UnsupportedProtocolVersionException(string requested, IEnumerable<string> supported, string? message = null)
+        : base(message ?? $"Unsupported protocol version '{requested}'.", McpErrorCode.UnsupportedProtocolVersion)
+    {
+        Throw.IfNull(requested);
+        Throw.IfNull(supported);
+
+        Requested = requested;
+        Supported = new List<string>(supported);
+    }
+
+    /// <summary>Gets the protocol version the client requested.</summary>
+    public string Requested { get; }
+
+    /// <summary>Gets the protocol versions the server supports.</summary>
+    public IReadOnlyList<string> Supported { get; }
+
+    internal JsonNode CreateErrorDataNode()
+    {
+        var payload = new UnsupportedProtocolVersionErrorData
+        {
+            Requested = Requested,
+            Supported = (IList<string>)Supported,
+        };
+
+        return JsonSerializer.SerializeToNode(payload, McpJsonUtilities.JsonContext.Default.UnsupportedProtocolVersionErrorData)!;
+    }
+
+    internal static bool TryCreateFromError(
+        string formattedMessage,
+        JsonRpcErrorDetail detail,
+        [NotNullWhen(true)] out UnsupportedProtocolVersionException? exception)
+    {
+        exception = null;
+
+        if (detail.Data is not JsonElement dataElement || dataElement.ValueKind is not JsonValueKind.Object)
+        {
+            return false;
+        }
+
+        var payload = dataElement.Deserialize(McpJsonUtilities.JsonContext.Default.UnsupportedProtocolVersionErrorData);
+        if (payload is null)
+        {
+            return false;
+        }
+
+        exception = new UnsupportedProtocolVersionException(payload.Requested, payload.Supported, formattedMessage);
+        return true;
+    }
+}