Automatically fall back from Streamable HTTP to SSE on the client by default (#456)

tests/ModelContextProtocol.AspNetCore.Tests/HttpServerIntegrationTests.cs

@@ -56,7 +56,7 @@ public abstract class HttpServerIntegrationTests : LoggedTest, IClassFixture<Sse
 
     [Fact]
     public async Task ListTools_Sse_TestServer()
-    {        
+    {
         // arrange
 
         // act

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs

@@ -1,5 +1,6 @@
 using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.DependencyInjection;
+using ModelContextProtocol.Client;
 
 namespace ModelContextProtocol.AspNetCore.Tests;
 
@@ -34,4 +35,112 @@ public class MapMcpStreamableHttpTests(ITestOutputHelper outputHelper) : MapMcpT
 
         Assert.Equal("TestCustomRouteServer", mcpClient.ServerInfo.Name);
     }
+
+    [Fact]
+    public async Task StreamableHttpMode_Works_WithRootEndpoint()
+    {
+        Builder.Services.AddMcpServer(options =>
+        {
+            options.ServerInfo = new()
+            {
+                Name = "StreamableHttpTestServer",
+                Version = "1.0.0",
+            };
+        }).WithHttpTransport(ConfigureStateless);
+        await using var app = Builder.Build();
+
+        app.MapMcp();
+
+        await app.StartAsync(TestContext.Current.CancellationToken);
+
+        await using var mcpClient = await ConnectAsync("/", new()
+        {
+            Endpoint = new Uri("http://localhost/"),
+            TransportMode = HttpTransportMode.AutoDetect
+        });
+
+        Assert.Equal("StreamableHttpTestServer", mcpClient.ServerInfo.Name);
+    }
+
+    [Fact]
+    public async Task AutoDetectMode_Works_WithRootEndpoint()
+    {
+        Builder.Services.AddMcpServer(options =>
+        {
+            options.ServerInfo = new()
+            {
+                Name = "AutoDetectTestServer",
+                Version = "1.0.0",
+            };
+        }).WithHttpTransport(ConfigureStateless);
+        await using var app = Builder.Build();
+
+        app.MapMcp();
+
+        await app.StartAsync(TestContext.Current.CancellationToken);
+
+        await using var mcpClient = await ConnectAsync("/", new()
+        {
+            Endpoint = new Uri("http://localhost/"),
+            TransportMode = HttpTransportMode.AutoDetect
+        });
+
+        Assert.Equal("AutoDetectTestServer", mcpClient.ServerInfo.Name);
+    }
+
+    [Fact]
+    public async Task AutoDetectMode_Works_WithSseEndpoint()
+    {
+        Assert.SkipWhen(Stateless, "SSE endpoint is disabled in stateless mode.");
+
+        Builder.Services.AddMcpServer(options =>
+        {
+            options.ServerInfo = new()
+            {
+                Name = "AutoDetectSseTestServer",
+                Version = "1.0.0",
+            };
+        }).WithHttpTransport(ConfigureStateless);
+        await using var app = Builder.Build();
+
+        app.MapMcp();
+
+        await app.StartAsync(TestContext.Current.CancellationToken);
+
+        await using var mcpClient = await ConnectAsync("/sse", new()
+        {
+            Endpoint = new Uri("http://localhost/sse"),
+            TransportMode = HttpTransportMode.AutoDetect
+        });
+
+        Assert.Equal("AutoDetectSseTestServer", mcpClient.ServerInfo.Name);
+    }
+
+    [Fact]
+    public async Task SseMode_Works_WithSseEndpoint()
+    {
+        Assert.SkipWhen(Stateless, "SSE endpoint is disabled in stateless mode.");
+
+        Builder.Services.AddMcpServer(options =>
+        {
+            options.ServerInfo = new()
+            {
+                Name = "SseTestServer",
+                Version = "1.0.0",
+            };
+        }).WithHttpTransport(ConfigureStateless);
+        await using var app = Builder.Build();
+
+        app.MapMcp();
+
+        await app.StartAsync(TestContext.Current.CancellationToken);
+
+        await using var mcpClient = await ConnectAsync(options: new()
+        {
+            Endpoint = new Uri("http://localhost/sse"),
+            TransportMode = HttpTransportMode.Sse
+        });
+
+        Assert.Equal("SseTestServer", mcpClient.ServerInfo.Name);
+    }
 }

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpTests.cs

@@ -20,16 +20,17 @@ public abstract class MapMcpTests(ITestOutputHelper testOutputHelper) : KestrelI
         options.Stateless = Stateless;
     }
 
-    protected async Task<IMcpClient> ConnectAsync(string? path = null)
+    protected async Task<IMcpClient> ConnectAsync(string? path = null, SseClientTransportOptions? options = null)
     {
+        // Default behavior when no options are provided
         path ??= UseStreamableHttp ? "/" : "/sse";
 
-        var sseClientTransportOptions = new SseClientTransportOptions()
+        await using var transport = new SseClientTransport(options ?? new SseClientTransportOptions()
         {
             Endpoint = new Uri($"http://localhost{path}"),
-            UseStreamableHttp = UseStreamableHttp,
-        };
-        await using var transport = new SseClientTransport(sseClientTransportOptions, HttpClient, LoggerFactory);
+            TransportMode = UseStreamableHttp ? HttpTransportMode.StreamableHttp : HttpTransportMode.Sse,
+        }, HttpClient, LoggerFactory);
+
         return await McpClientFactory.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
     }
 

tests/ModelContextProtocol.AspNetCore.Tests/StatelessServerIntegrationTests.cs

@@ -9,6 +9,6 @@ public class StatelessServerIntegrationTests(SseServerIntegrationTestFixture fix
     {
         Endpoint = new Uri("http://localhost/stateless"),
         Name = "In-memory Streamable HTTP Client",
-        UseStreamableHttp = true,
+        TransportMode = HttpTransportMode.StreamableHttp,
     };
 }

tests/ModelContextProtocol.AspNetCore.Tests/StatelessServerTests.cs

@@ -18,7 +18,7 @@ public class StatelessServerTests(ITestOutputHelper outputHelper) : KestrelInMem
     {
         Endpoint = new Uri("http://localhost/"),
         Name = "In-memory Streamable HTTP Client",
-        UseStreamableHttp = true,
+        TransportMode = HttpTransportMode.StreamableHttp,
     };
 
     private async Task StartAsync()

tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpClientConformanceTests.cs

@@ -98,7 +98,7 @@ public class StreamableHttpClientConformanceTests(ITestOutputHelper outputHelper
         await using var transport = new SseClientTransport(new()
         {
             Endpoint = new("http://localhost/mcp"),
-            UseStreamableHttp = true,
+            TransportMode = HttpTransportMode.StreamableHttp,
         }, HttpClient, LoggerFactory);
 
         await using var client = await McpClientFactory.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);
@@ -118,7 +118,7 @@ public class StreamableHttpClientConformanceTests(ITestOutputHelper outputHelper
         await using var transport = new SseClientTransport(new()
         {
             Endpoint = new("http://localhost/mcp"),
-            UseStreamableHttp = true,
+            TransportMode = HttpTransportMode.StreamableHttp,
         }, HttpClient, LoggerFactory);
 
         await using var client = await McpClientFactory.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken);

tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpServerIntegrationTests.cs

@@ -15,7 +15,7 @@ public class StreamableHttpServerIntegrationTests(SseServerIntegrationTestFixtur
     {
         Endpoint = new Uri("http://localhost/"),
         Name = "In-memory Streamable HTTP Client",
-        UseStreamableHttp = true,
+        TransportMode = HttpTransportMode.StreamableHttp,
     };
 
     [Fact]