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

src/ModelContextProtocol.Core/MissingRequiredClientCapabilityException.cs

@@ -0,0 +1,67 @@
+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 requires a client capability that was not declared
+/// in the request's per-request <c>_meta/io.modelcontextprotocol/clientCapabilities</c> field.
+/// </summary>
+/// <remarks>
+/// Introduced by the draft protocol revision (SEP-2575). Servers throw this exception when a handler cannot
+/// proceed because the client did not declare a required capability for the request. The exception is converted
+/// to a JSON-RPC error response with code <see cref="McpErrorCode.MissingRequiredClientCapability"/> (<c>-32003</c>)
+/// and a <see cref="MissingRequiredClientCapabilityErrorData"/> payload.
+/// </remarks>
+public sealed class MissingRequiredClientCapabilityException : McpProtocolException
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="MissingRequiredClientCapabilityException"/> class.
+    /// </summary>
+    /// <param name="requiredCapabilities">The capabilities the server requires for the request.</param>
+    /// <param name="message">A human-readable description of the error. If <see langword="null"/>, a default message is used.</param>
+    public MissingRequiredClientCapabilityException(ClientCapabilities requiredCapabilities, string? message = null)
+        : base(message ?? "The request requires client capabilities that were not declared in _meta/clientCapabilities.",
+               McpErrorCode.MissingRequiredClientCapability)
+    {
+        Throw.IfNull(requiredCapabilities);
+        RequiredCapabilities = requiredCapabilities;
+    }
+
+    /// <summary>Gets the client capabilities required for the request.</summary>
+    public ClientCapabilities RequiredCapabilities { get; }
+
+    internal JsonNode CreateErrorDataNode()
+    {
+        var payload = new MissingRequiredClientCapabilityErrorData
+        {
+            RequiredCapabilities = RequiredCapabilities,
+        };
+
+        return JsonSerializer.SerializeToNode(payload, McpJsonUtilities.JsonContext.Default.MissingRequiredClientCapabilityErrorData)!;
+    }
+
+    internal static bool TryCreateFromError(
+        string formattedMessage,
+        JsonRpcErrorDetail detail,
+        [NotNullWhen(true)] out MissingRequiredClientCapabilityException? 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.MissingRequiredClientCapabilityErrorData);
+        if (payload?.RequiredCapabilities is null)
+        {
+            return false;
+        }
+
+        exception = new MissingRequiredClientCapabilityException(payload.RequiredCapabilities, formattedMessage);
+        return true;
+    }
+}