Keep AutoDetect channel open after provisional SSE failure

src/ModelContextProtocol.Core/Client/SseClientSessionTransport.cs

@@ -23,6 +23,7 @@ internal sealed partial class SseClientSessionTransport : TransportBase
     private Task? _receiveTask;
     private readonly ILogger _logger;
     private readonly TaskCompletionSource<bool> _connectionEstablished;
+    private volatile bool _sseAdopted;
 
     /// <summary>
     /// SSE transport for a single session. Unlike stdio it does not launch a process, but connects to an existing server.
@@ -107,6 +108,8 @@ public override async Task SendMessageAsync(
 
             throw HttpResponseMessageExtensions.CreateHttpRequestException(response, responseBody);
         }
+
+        _sseAdopted = true;
     }
 
     private async Task CloseAsync()
@@ -129,7 +132,7 @@ private async Task CloseAsync()
         }
         finally
         {
-            SetDisconnected(new ClientTransportClosedException(new HttpClientCompletionDetails()));
+            SetSseDisconnected(new ClientTransportClosedException(new HttpClientCompletionDetails()));
         }
     }
 
@@ -190,7 +193,7 @@ private async Task ReceiveMessagesAsync(CancellationToken cancellationToken)
             }
             else
             {
-                SetDisconnected(new ClientTransportClosedException(new HttpClientCompletionDetails
+                SetSseDisconnected(new ClientTransportClosedException(new HttpClientCompletionDetails
                 {
                     HttpStatusCode = failureStatusCode,
                     Exception = ex,
@@ -202,7 +205,18 @@ private async Task ReceiveMessagesAsync(CancellationToken cancellationToken)
         }
         finally
         {
-            SetDisconnected(new ClientTransportClosedException(new HttpClientCompletionDetails()));
+            SetSseDisconnected(new ClientTransportClosedException(new HttpClientCompletionDetails()));
+        }
+    }
+
+    private void SetSseDisconnected(Exception error)
+    {
+        // If AutoDetect is still probing SSE, leave its shared message channel open so it can
+        // retry with another transport. A successful POST means SSE was selected and owns the
+        // channel from that point on, matching Streamable HTTP's adoption behavior.
+        if (_options.TransportMode is not HttpTransportMode.AutoDetect || _sseAdopted)
+        {
+            SetDisconnected(error);
         }
     }
 

tests/ModelContextProtocol.Tests/Transport/HttpClientTransportAutoDetectTests.cs

@@ -2,6 +2,7 @@
 using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Tests.Utils;
 using Microsoft.Extensions.Logging;
+using System.IO.Pipelines;
 using System.Net;
 
 namespace ModelContextProtocol.Tests.Transport;
@@ -163,6 +164,115 @@ public async Task AutoDetectMode_FallsBackToSse_WhenStreamableHttpFails()
         Assert.NotNull(session);
     }
 
+    [Fact]
+    public async Task AutoDetectMode_WhenProvisionalSseFails_LeavesSharedMessageChannelOpen()
+    {
+        var options = new HttpClientTransportOptions
+        {
+            Endpoint = new Uri("http://localhost"),
+            TransportMode = HttpTransportMode.AutoDetect,
+            Name = "AutoDetect shared channel test client"
+        };
+
+        using var mockHttpHandler = new MockHttpHandler();
+        using var httpClient = new HttpClient(mockHttpHandler);
+        await using var transport = new HttpClientTransport(options, httpClient, LoggerFactory);
+        var streamableHttpPostCount = 0;
+
+        mockHttpHandler.RequestHandler = request =>
+        {
+            if (request.Method == HttpMethod.Get)
+            {
+                return Task.FromResult(new HttpResponseMessage(HttpStatusCode.MethodNotAllowed));
+            }
+
+            if (request.Method == HttpMethod.Post && ++streamableHttpPostCount == 1)
+            {
+                return Task.FromResult(new HttpResponseMessage(HttpStatusCode.NotFound)
+                {
+                    Content = new StringContent("Invalid session ID"),
+                });
+            }
+
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
+            {
+                Content = new StringContent(
+                    "{\"jsonrpc\":\"2.0\",\"id\":2,\"result\":{\"protocolVersion\":\"2025-11-25\",\"capabilities\":{},\"serverInfo\":{\"name\":\"test\",\"version\":\"1.0\"}}}",
+                    System.Text.Encoding.UTF8,
+                    "application/json"),
+            });
+        };
+
+        await using var session = await transport.ConnectAsync(TestContext.Current.CancellationToken);
+
+        await Assert.ThrowsAsync<HttpRequestException>(() =>
+            session.SendMessageAsync(
+                new JsonRpcRequest { Method = RequestMethods.ServerDiscover, Id = new RequestId(1) },
+                TestContext.Current.CancellationToken));
+
+        await session.SendMessageAsync(
+            new JsonRpcRequest { Method = RequestMethods.Initialize, Id = new RequestId(2) },
+            TestContext.Current.CancellationToken);
+
+        var response = await session.MessageReader.ReadAsync(TestContext.Current.CancellationToken);
+        Assert.Equal(new RequestId(2), Assert.IsType<JsonRpcResponse>(response).Id);
+    }
+
+    [Fact]
+    public async Task AutoDetectMode_WhenAdoptedSseDisconnects_CompletesSharedMessageChannel()
+    {
+        var options = new HttpClientTransportOptions
+        {
+            Endpoint = new Uri("http://localhost"),
+            TransportMode = HttpTransportMode.AutoDetect,
+            Name = "AutoDetect adopted SSE test client"
+        };
+
+        using var mockHttpHandler = new MockHttpHandler();
+        using var httpClient = new HttpClient(mockHttpHandler);
+        await using var transport = new HttpClientTransport(options, httpClient, LoggerFactory);
+        var ssePipe = new Pipe();
+        var postCount = 0;
+
+        await ssePipe.Writer.WriteAsync(
+            System.Text.Encoding.UTF8.GetBytes("event: endpoint\r\ndata: /sse-endpoint\r\n\r\n"),
+            TestContext.Current.CancellationToken);
+
+        mockHttpHandler.RequestHandler = request =>
+        {
+            if (request.Method == HttpMethod.Get)
+            {
+                var content = new StreamContent(ssePipe.Reader.AsStream());
+                content.Headers.ContentType = new("text/event-stream");
+                return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK) { Content = content });
+            }
+
+            if (request.Method == HttpMethod.Post && ++postCount == 1)
+            {
+                return Task.FromResult(new HttpResponseMessage(HttpStatusCode.NotFound)
+                {
+                    Content = new StringContent("Streamable HTTP not supported"),
+                });
+            }
+
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK));
+        };
+
+        await using var session = await transport.ConnectAsync(TestContext.Current.CancellationToken);
+
+        await session.SendMessageAsync(
+            new JsonRpcRequest { Method = RequestMethods.Initialize, Id = new RequestId(1) },
+            TestContext.Current.CancellationToken);
+
+        await ssePipe.Writer.CompleteAsync();
+
+        var exception = await Assert.ThrowsAsync<ClientTransportClosedException>(
+            async () => await session.MessageReader.Completion.WaitAsync(
+                TestConstants.DefaultTimeout,
+                TestContext.Current.CancellationToken));
+        Assert.IsType<HttpClientCompletionDetails>(exception.Details);
+    }
+
     // Regression test for https://github.com/modelcontextprotocol/csharp-sdk/issues/1526
     // When Streamable HTTP returns 415 (e.g. wrong Content-Type) and the SSE fallback also fails
     // (e.g. a Streamable-HTTP-only server returns 405 to the GET), the surfaced exception must