Write "event: message" to SSE response

Author: halter73

src/ModelContextProtocol/Protocol/Transport/SseResponseStreamTransport.cs

@@ -76,7 +76,8 @@ public async Task SendMessageAsync(IJsonRpcMessage message, CancellationToken ca
             throw new InvalidOperationException($"Transport is not connected. Make sure to call {nameof(RunAsync)} first.");
         }
 
-        await _outgoingSseChannel.Writer.WriteAsync(new SseItem<IJsonRpcMessage?>(message), cancellationToken);
+        // Emit redundant "event: message" lines for better compatibility with other SDKs.
+        await _outgoingSseChannel.Writer.WriteAsync(new SseItem<IJsonRpcMessage?>(message, "message"), cancellationToken);
     }
 
     /// <summary>

tests/ModelContextProtocol.Tests/SseServerIntegrationTests.cs

@@ -1,6 +1,8 @@
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol.Types;
 using ModelContextProtocol.Tests.Utils;
+using System.Net;
+using System.Text;
 
 namespace ModelContextProtocol.Tests;
 
@@ -30,6 +32,12 @@ private Task<IMcpClient> GetClientAsync(McpClientOptions? options = null)
             cancellationToken: TestContext.Current.CancellationToken);
     }
 
+    private HttpClient GetHttpClient() =>
+        new()
+        {
+            BaseAddress = new(_fixture.DefaultConfig.Location!),
+        };
+
     [Fact]
     public async Task ConnectAndPing_Sse_TestServer()
     {
@@ -271,4 +279,46 @@ public async Task CallTool_Sse_EchoServer_Concurrently()
             Assert.Equal($"Echo: Hello MCP! {i}", textContent.Text);
         }
     }
+
+    [Fact]
+    public async Task EventSourceStream_Includes_MessageEventType()
+    {
+        // Simulate our own MCP client handshake using a plain HttpClient so we can look for "event: message"
+        // in the raw SSE response stream which is not exposed by the real MCP client.
+        using var httpClient = GetHttpClient();
+        await using var sseResponse = await httpClient.GetStreamAsync("", TestContext.Current.CancellationToken);
+        using var streamReader = new StreamReader(sseResponse);
+
+        // read event stream from the server
+        var endpointEvent = await streamReader.ReadLineAsync(TestContext.Current.CancellationToken);
+        Assert.Equal("event: endpoint", endpointEvent);
+
+        var endpointData = await streamReader.ReadLineAsync(TestContext.Current.CancellationToken);
+        Assert.NotNull(endpointData);
+        Assert.StartsWith("data: ", endpointData);
+        var messageEndpoint = endpointData["data: ".Length..];
+
+        const string initializeRequest = """
+            {"jsonrpc":"2.0","id":"1","method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"IntegrationTestClient","version":"1.0.0"}}}
+            """;
+        using (var initializeRequestBody = new StringContent(initializeRequest, Encoding.UTF8, "application/json"))
+        {
+            var response = await httpClient.PostAsync(messageEndpoint, initializeRequestBody, TestContext.Current.CancellationToken);
+            Assert.Equal(HttpStatusCode.Accepted, response.StatusCode);
+        }
+
+        const string initializeNotification = """
+            {"jsonrpc":"2.0","method":"notifications/initialized"}
+            """;
+        using (var initializeNotificationBody = new StringContent(initializeNotification, Encoding.UTF8, "application/json"))
+        {
+            var response = await httpClient.PostAsync(messageEndpoint, initializeNotificationBody, TestContext.Current.CancellationToken);
+            Assert.Equal(HttpStatusCode.Accepted, response.StatusCode);
+        }
+
+        // read event stream from the server
+        Assert.Equal("", await streamReader.ReadLineAsync(TestContext.Current.CancellationToken));
+        var messageEvent = await streamReader.ReadLineAsync(TestContext.Current.CancellationToken);
+        Assert.Equal("event: message", messageEvent);
+    }
 }