Add support for data to McpProtocolException in JSON-RPC errors

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/McpSessionHandler.cs

@@ -186,10 +186,10 @@ ex is OperationCanceledException &&
                                 {
                                     Code = (int)mcpProtocolException.ErrorCode,
                                     Message = mcpProtocolException.Message,
+                                    Data = ConvertExceptionData(mcpProtocolException.Data),
                                 } : ex is McpException mcpException ?
                                 new()
                                 {
-
                                     Code = (int)McpErrorCode.InternalError,
                                     Message = mcpException.Message,
                                 } :
@@ -769,6 +769,29 @@ private static TimeSpan GetElapsed(long startingTimestamp) =>
         return null;
     }
 
+    /// <summary>
+    /// Converts the Exception.Data dictionary to a serializable Dictionary&lt;string, object?&gt;.
+    /// Returns null if the data dictionary is empty.
+    /// </summary>
+    private static Dictionary<string, object?>? ConvertExceptionData(System.Collections.IDictionary data)
+    {
+        if (data.Count == 0)
+        {
+            return null;
+        }
+
+        var result = new Dictionary<string, object?>(data.Count);
+        foreach (System.Collections.DictionaryEntry entry in data)
+        {
+            if (entry.Key is string key)
+            {
+                result[key] = entry.Value;
+            }
+        }
+
+        return result.Count > 0 ? result : null;
+    }
+
     [LoggerMessage(Level = LogLevel.Information, Message = "{EndpointName} message processing canceled.")]
     private partial void LogEndpointMessageProcessingCanceled(string endpointName);
 

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -671,6 +671,66 @@ await transport.SendMessageAsync(
         await runTask;
     }
 
+    [Fact]
+    public async Task Can_Handle_Call_Tool_Requests_With_McpProtocolException_And_Data()
+    {
+        const string errorMessage = "Resource not found";
+        const McpErrorCode errorCode = (McpErrorCode)(-32002);
+        const string resourceUri = "file:///path/to/resource";
+
+        await using var transport = new TestServerTransport();
+        var options = CreateOptions(new ServerCapabilities { Tools = new() });
+        options.Handlers.CallToolHandler = async (request, ct) =>
+        {
+            throw new McpProtocolException(errorMessage, errorCode)
+            {
+                Data =
+                {
+                    { "uri", resourceUri }
+                }
+            };
+        };
+        options.Handlers.ListToolsHandler = (request, ct) => throw new NotImplementedException();
+
+        await using var server = McpServer.Create(transport, options, LoggerFactory);
+
+        var runTask = server.RunAsync(TestContext.Current.CancellationToken);
+
+        var receivedMessage = new TaskCompletionSource<JsonRpcError>();
+
+        transport.OnMessageSent = (message) =>
+        {
+            if (message is JsonRpcError error && error.Id.ToString() == "55")
+                receivedMessage.SetResult(error);
+        };
+
+        await transport.SendMessageAsync(
+            new JsonRpcRequest
+            {
+                Method = RequestMethods.ToolsCall,
+                Id = new RequestId(55)
+            },
+            TestContext.Current.CancellationToken
+        );
+
+        var error = await receivedMessage.Task.WaitAsync(TimeSpan.FromSeconds(10), TestContext.Current.CancellationToken);
+        Assert.NotNull(error);
+        Assert.NotNull(error.Error);
+        Assert.Equal((int)errorCode, error.Error.Code);
+        Assert.Equal(errorMessage, error.Error.Message);
+        Assert.NotNull(error.Error.Data);
+
+        // Verify the data contains the uri
+        var dataJson = JsonSerializer.Serialize(error.Error.Data, McpJsonUtilities.DefaultOptions);
+        var dataObject = JsonSerializer.Deserialize<JsonObject>(dataJson, McpJsonUtilities.DefaultOptions);
+        Assert.NotNull(dataObject);
+        Assert.True(dataObject.ContainsKey("uri"));
+        Assert.Equal(resourceUri, dataObject["uri"]?.GetValue<string>());
+
+        await transport.DisposeAsync();
+        await runTask;
+    }
+
     private async Task Can_Handle_Requests(ServerCapabilities? serverCapabilities, string method, Action<McpServerOptions>? configureOptions, Action<McpServer, JsonNode?> assertResult)
     {
         await using var transport = new TestServerTransport();