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

src/ModelContextProtocol/Client/AutoDetectingClientSessionTransport.cs

@@ -0,0 +1,143 @@
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using ModelContextProtocol.Protocol;
+using System.Net;
+using System.Threading.Channels;
+
+namespace ModelContextProtocol.Client;
+
+/// <summary>
+/// A transport that automatically detects whether to use Streamable HTTP or SSE transport
+/// by trying Streamable HTTP first and falling back to SSE if that fails.
+/// </summary>
+internal sealed partial class AutoDetectingClientSessionTransport : ITransport
+{
+    private readonly SseClientTransportOptions _options;
+    private readonly HttpClient _httpClient;
+    private readonly ILoggerFactory? _loggerFactory;
+    private readonly ILogger _logger;
+    private readonly string _name;
+    private readonly Channel<JsonRpcMessage> _messageChannel;
+
+    public AutoDetectingClientSessionTransport(SseClientTransportOptions transportOptions, HttpClient httpClient, ILoggerFactory? loggerFactory, string endpointName)
+    {
+        Throw.IfNull(transportOptions);
+        Throw.IfNull(httpClient);
+
+        _options = transportOptions;
+        _httpClient = httpClient;
+        _loggerFactory = loggerFactory;
+        _logger = (ILogger?)loggerFactory?.CreateLogger<AutoDetectingClientSessionTransport>() ?? NullLogger.Instance;
+        _name = endpointName;
+
+        // Same as TransportBase.cs.
+        _messageChannel = Channel.CreateUnbounded<JsonRpcMessage>(new UnboundedChannelOptions
+        {
+            SingleReader = true,
+            SingleWriter = false,
+        });
+    }
+
+    /// <summary>
+    /// Returns the active transport (either StreamableHttp or SSE)
+    /// </summary>
+    internal ITransport? ActiveTransport { get; private set; }
+
+    public ChannelReader<JsonRpcMessage> MessageReader => _messageChannel.Reader;
+
+    /// <inheritdoc/>
+    public Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
+    {
+        if (ActiveTransport is null)
+        {
+            return InitializeAsync(message, cancellationToken);
+        }
+
+        return ActiveTransport.SendMessageAsync(message, cancellationToken);
+    }
+
+    private async Task InitializeAsync(JsonRpcMessage message, CancellationToken cancellationToken)
+    {
+        // Try StreamableHttp first
+        var streamableHttpTransport = new StreamableHttpClientSessionTransport(_name, _options, _httpClient, _messageChannel, _loggerFactory);
+
+        try
+        {
+            LogAttemptingStreamableHttp(_name);
+            using var response = await streamableHttpTransport.SendHttpRequestAsync(message, cancellationToken).ConfigureAwait(false);
+
+            if (response.IsSuccessStatusCode)
+            {
+                LogUsingStreamableHttp(_name);
+                ActiveTransport = streamableHttpTransport;
+            }
+            else
+            {
+                // If the status code is not success, fall back to SSE
+                LogStreamableHttpFailed(_name, response.StatusCode);
+
+                await streamableHttpTransport.DisposeAsync().ConfigureAwait(false);
+                await InitializeSseTransportAsync(message, cancellationToken).ConfigureAwait(false);
+            }
+        }
+        catch
+        {
+            // If nothing threw inside the try block, we've either set streamableHttpTransport as the
+            // ActiveTransport, or else we will have disposed it in the !IsSuccessStatusCode else block.
+            await streamableHttpTransport.DisposeAsync().ConfigureAwait(false);
+            throw;
+        }
+    }
+
+    private async Task InitializeSseTransportAsync(JsonRpcMessage message, CancellationToken cancellationToken)
+    {
+        var sseTransport = new SseClientSessionTransport(_name, _options, _httpClient, _messageChannel, _loggerFactory);
+
+        try
+        {
+            LogAttemptingSSE(_name);
+            await sseTransport.ConnectAsync(cancellationToken).ConfigureAwait(false);
+            await sseTransport.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
+
+            LogUsingSSE(_name);
+            ActiveTransport = sseTransport;
+        }
+        catch
+        {
+            await sseTransport.DisposeAsync().ConfigureAwait(false);
+            throw;
+        }
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        try
+        {
+            if (ActiveTransport is not null)
+            {
+                await ActiveTransport.DisposeAsync().ConfigureAwait(false);
+            }
+        }
+        finally
+        {
+            // In the majority of cases, either the Streamable HTTP transport or SSE transport has completed the channel by now.
+            // However, this may not be the case if HttpClient throws during the initial request due to misconfiguration.
+            _messageChannel.Writer.TryComplete();
+        }
+    }
+
+    [LoggerMessage(Level = LogLevel.Debug, Message = "{EndpointName} attempting to connect using Streamable HTTP transport.")]
+    private partial void LogAttemptingStreamableHttp(string endpointName);
+
+    [LoggerMessage(Level = LogLevel.Information, Message = "{EndpointName} streamable HTTP transport failed with status code {StatusCode}, falling back to SSE transport.")]
+    private partial void LogStreamableHttpFailed(string endpointName, HttpStatusCode statusCode);
+
+    [LoggerMessage(Level = LogLevel.Information, Message = "{EndpointName} using Streamable HTTP transport.")]
+    private partial void LogUsingStreamableHttp(string endpointName);
+
+    [LoggerMessage(Level = LogLevel.Debug, Message = "{EndpointName} attempting to connect using SSE transport.")]
+    private partial void LogAttemptingSSE(string endpointName);
+
+    [LoggerMessage(Level = LogLevel.Information, Message = "{EndpointName} using SSE transport.")]
+    private partial void LogUsingSSE(string endpointName);
+}

src/ModelContextProtocol/Client/HttpTransportMode.cs

@@ -0,0 +1,23 @@
+namespace ModelContextProtocol.Client;
+
+/// <summary>
+/// Specifies the transport mode for HTTP client connections.
+/// </summary>
+public enum HttpTransportMode
+{
+    /// <summary>
+    /// Automatically detect the appropriate transport by trying Streamable HTTP first, then falling back to SSE if that fails.
+    /// This is the recommended mode for maximum compatibility.
+    /// </summary>
+    AutoDetect,
+
+    /// <summary>
+    /// Use only the Streamable HTTP transport.
+    /// </summary>
+    StreamableHttp,
+
+    /// <summary>
+    /// Use only the HTTP with SSE transport.
+    /// </summary>
+    Sse
+}

src/ModelContextProtocol/Client/SseClientSessionTransport.cs

@@ -6,6 +6,7 @@ using System.Net.Http.Headers;
 using System.Net.ServerSentEvents;
 using System.Text;
 using System.Text.Json;
+using System.Threading.Channels;
 
 namespace ModelContextProtocol.Client;
 
@@ -24,15 +25,16 @@ internal sealed partial class SseClientSessionTransport : TransportBase
     private readonly TaskCompletionSource<bool> _connectionEstablished;
 
     /// <summary>
-    /// SSE transport for client endpoints. Unlike stdio it does not launch a process, but connects to an existing server.
+    /// SSE transport for a single session. Unlike stdio it does not launch a process, but connects to an existing server.
     /// The HTTP server can be local or remote, and must support the SSE protocol.
     /// </summary>
-    /// <param name="transportOptions">Configuration options for the transport.</param>
-    /// <param name="httpClient">The HTTP client instance used for requests.</param>
-    /// <param name="loggerFactory">Logger factory for creating loggers.</param>
-    /// <param name="endpointName">The endpoint name used for logging purposes.</param>
-    public SseClientSessionTransport(SseClientTransportOptions transportOptions, HttpClient httpClient, ILoggerFactory? loggerFactory, string endpointName)
-        : base(endpointName, loggerFactory)
+    public SseClientSessionTransport(
+        string endpointName,
+        SseClientTransportOptions transportOptions,
+        HttpClient httpClient,
+        Channel<JsonRpcMessage>? messageChannel,
+        ILoggerFactory? loggerFactory)
+        : base(endpointName, messageChannel, loggerFactory)
     {
         Throw.IfNull(transportOptions);
         Throw.IfNull(httpClient);

src/ModelContextProtocol/Client/SseClientTransport.cs

@@ -4,11 +4,11 @@ using ModelContextProtocol.Protocol;
 namespace ModelContextProtocol.Client;
 
 /// <summary>
-/// Provides an <see cref="IClientTransport"/> over HTTP using the Server-Sent Events (SSE) protocol.
+/// Provides an <see cref="IClientTransport"/> over HTTP using the Server-Sent Events (SSE) or Streamable HTTP protocol.
 /// </summary>
 /// <remarks>
-/// This transport connects to an MCP server over HTTP using SSE,
-/// allowing for real-time server-to-client communication with a standard HTTP request.
+/// This transport connects to an MCP server over HTTP using SSE or Streamable HTTP,
+/// allowing for real-time server-to-client communication with a standard HTTP requests.
 /// Unlike the <see cref="StdioClientTransport"/>, this transport connects to an existing server
 /// rather than launching a new process.
 /// </remarks>
@@ -36,7 +36,7 @@ public sealed class SseClientTransport : IClientTransport, IAsyncDisposable
     /// <param name="httpClient">The HTTP client instance used for requests.</param>
     /// <param name="loggerFactory">Logger factory for creating loggers used for diagnostic output during transport operations.</param>
     /// <param name="ownsHttpClient">
-    /// <see langword="true"/> to dispose of <paramref name="httpClient"/> when the transport is disposed; 
+    /// <see langword="true"/> to dispose of <paramref name="httpClient"/> when the transport is disposed;
     /// <see langword="false"/> if the caller is retaining ownership of the <paramref name="httpClient"/>'s lifetime.
     /// </param>
     public SseClientTransport(SseClientTransportOptions transportOptions, HttpClient httpClient, ILoggerFactory? loggerFactory = null, bool ownsHttpClient = false)
@@ -57,12 +57,22 @@ public sealed class SseClientTransport : IClientTransport, IAsyncDisposable
     /// <inheritdoc />
     public async Task<ITransport> ConnectAsync(CancellationToken cancellationToken = default)
     {
-        if (_options.UseStreamableHttp)
+        switch (_options.TransportMode)
         {
-            return new StreamableHttpClientSessionTransport(_options, _httpClient, _loggerFactory, Name);
+            case HttpTransportMode.AutoDetect:
+                return new AutoDetectingClientSessionTransport(_options, _httpClient, _loggerFactory, Name);
+            case HttpTransportMode.StreamableHttp:
+                return new StreamableHttpClientSessionTransport(Name, _options, _httpClient, messageChannel: null, _loggerFactory);
+            case HttpTransportMode.Sse:
+                return await ConnectSseTransportAsync(cancellationToken).ConfigureAwait(false);
+            default:
+                throw new InvalidOperationException($"Unsupported transport mode: {_options.TransportMode}");
         }
+    }
 
-        var sessionTransport = new SseClientSessionTransport(_options, _httpClient, _loggerFactory, Name);
+    private async Task<ITransport> ConnectSseTransportAsync(CancellationToken cancellationToken)
+    {
+        var sessionTransport = new SseClientSessionTransport(Name, _options, _httpClient, messageChannel: null, _loggerFactory);
 
         try
         {

src/ModelContextProtocol/Client/SseClientTransportOptions.cs

@@ -31,11 +31,19 @@ public record SseClientTransportOptions
     }
 
     /// <summary>
-    /// Gets or sets a value indicating whether to use "Streamable HTTP" for the transport rather than "HTTP with SSE". Defaults to false.
+    /// Gets or sets the transport mode to use for the connection. Defaults to <see cref="HttpTransportMode.AutoDetect"/>.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// When set to <see cref="HttpTransportMode.AutoDetect"/> (the default), the client will first attempt to use
+    /// Streamable HTTP transport and automatically fall back to SSE transport if the server doesn't support it.
+    /// </para>
+    /// <para>
     /// <see href="https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#streamable-http">Streamable HTTP transport specification</see>.
     /// <see href="https://modelcontextprotocol.io/specification/2024-11-05/basic/transports#http-with-sse">HTTP with SSE transport specification</see>.
-    /// </summary>
-    public bool UseStreamableHttp { get; init; }
+    /// </para>
+    /// </remarks>
+    public HttpTransportMode TransportMode { get; init; } = HttpTransportMode.AutoDetect;
 
     /// <summary>
     /// Gets a transport identifier used for logging purposes.

src/ModelContextProtocol/Client/StreamClientSessionTransport.cs

@@ -17,7 +17,7 @@ internal class StreamClientSessionTransport : TransportBase
     /// Initializes a new instance of the <see cref="StreamClientSessionTransport"/> class.
     /// </summary>
     /// <param name="serverInput">
-    /// The text writer connected to the server's input stream. 
+    /// The text writer connected to the server's input stream.
     /// Messages written to this writer will be sent to the server.
     /// </param>
     /// <param name="serverOutput">
@@ -41,17 +41,17 @@ internal class StreamClientSessionTransport : TransportBase
         _serverOutput = serverOutput;
         _serverInput = serverInput;
 
+        SetConnected();
+
         // Start reading messages in the background. We use the rarer pattern of new Task + Start
         // in order to ensure that the body of the task will always see _readTask initialized.
         // It is then able to reliably null it out on completion.
         var readTask = new Task<Task>(
-            thisRef => ((StreamClientSessionTransport)thisRef!).ReadMessagesAsync(_shutdownCts.Token), 
+            thisRef => ((StreamClientSessionTransport)thisRef!).ReadMessagesAsync(_shutdownCts.Token),
             this,
             TaskCreationOptions.DenyChildAttach);
         _readTask = readTask.Unwrap();
         readTask.Start();
-
-        SetConnected();
     }
 
     /// <inheritdoc/>
@@ -80,7 +80,7 @@ internal class StreamClientSessionTransport : TransportBase
     }
 
     /// <inheritdoc/>
-    public override ValueTask DisposeAsync() => 
+    public override ValueTask DisposeAsync() =>
         CleanupAsync(cancellationToken: CancellationToken.None);
 
     private async Task ReadMessagesAsync(CancellationToken cancellationToken)

src/ModelContextProtocol/Client/StreamableHttpClientSessionTransport.cs

@@ -4,6 +4,8 @@ using System.Net.Http.Headers;
 using System.Net.ServerSentEvents;
 using System.Text.Json;
 using ModelContextProtocol.Protocol;
+using System.Threading.Channels;
+
 #if NET
 using System.Net.Http.Json;
 #else
@@ -28,8 +30,13 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
     private string? _mcpSessionId;
     private Task? _getReceiveTask;
 
-    public StreamableHttpClientSessionTransport(SseClientTransportOptions transportOptions, HttpClient httpClient, ILoggerFactory? loggerFactory, string endpointName)
-        : base(endpointName, loggerFactory)
+    public StreamableHttpClientSessionTransport(
+        string endpointName,
+        SseClientTransportOptions transportOptions,
+        HttpClient httpClient,
+        Channel<JsonRpcMessage>? messageChannel,
+        ILoggerFactory? loggerFactory)
+        : base(endpointName, messageChannel, loggerFactory)
     {
         Throw.IfNull(transportOptions);
         Throw.IfNull(httpClient);
@@ -46,9 +53,15 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
     }
 
     /// <inheritdoc/>
-    public override async Task SendMessageAsync(
-        JsonRpcMessage message,
-        CancellationToken cancellationToken = default)
+    public override async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
+    {
+        // Immediately dispose the response. SendHttpRequestAsync only returns the response so the auto transport can look at it.
+        using var response = await SendHttpRequestAsync(message, cancellationToken).ConfigureAwait(false);
+        response.EnsureSuccessStatusCode();
+    }
+
+    // This is used by the auto transport so it can fall back and try SSE given a non-200 response without catching an exception.
+    internal async Task<HttpResponseMessage> SendHttpRequestAsync(JsonRpcMessage message, CancellationToken cancellationToken)
     {
         using var sendCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, _connectionCts.Token);
         cancellationToken = sendCts.Token;
@@ -73,9 +86,14 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
         };
 
         CopyAdditionalHeaders(httpRequestMessage.Headers, _options.AdditionalHeaders, _mcpSessionId);
-        using var response = await _httpClient.SendAsync(httpRequestMessage, HttpCompletionOption.ResponseHeadersRead, cancellationToken).ConfigureAwait(false);
 
-        response.EnsureSuccessStatusCode();
+        var response = await _httpClient.SendAsync(httpRequestMessage, HttpCompletionOption.ResponseHeadersRead, cancellationToken).ConfigureAwait(false);
+
+        // We'll let the caller decide whether to throw or fall back given an unsuccessful response.
+        if (!response.IsSuccessStatusCode)
+        {
+            return response;
+        }
 
         var rpcRequest = message as JsonRpcRequest;
         JsonRpcMessage? rpcResponseCandidate = null;
@@ -93,7 +111,7 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
 
         if (rpcRequest is null)
         {
-            return;
+            return response;
         }
 
         if (rpcResponseCandidate is not JsonRpcMessageWithId messageWithId || messageWithId.Id != rpcRequest.Id)
@@ -111,6 +129,8 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
 
             _getReceiveTask = ReceiveUnsolicitedMessagesAsync();
         }
+
+        return response;
     }
 
     public override async ValueTask DisposeAsync()
@@ -136,7 +156,12 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
         }
         finally
         {
-            SetDisconnected();
+            // If we're auto-detecting the transport and failed to connect, leave the message Channel open for the SSE transport.
+            // This class isn't directly exposed to public callers, so we don't have to worry about changing the _state in this case.
+            if (_options.TransportMode is not HttpTransportMode.AutoDetect || _getReceiveTask is not null)
+            {
+                SetDisconnected();
+            }
         }
     }
 

src/ModelContextProtocol/Protocol/TransportBase.cs

@@ -36,12 +36,20 @@ public abstract partial class TransportBase : ITransport
     /// Initializes a new instance of the <see cref="TransportBase"/> class.
     /// </summary>
     protected TransportBase(string name, ILoggerFactory? loggerFactory)
+        : this(name, null, loggerFactory)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="TransportBase"/> class with a specified channel to back <see cref="MessageReader"/>.
+    /// </summary>
+    internal TransportBase(string name, Channel<JsonRpcMessage>? messageChannel, ILoggerFactory? loggerFactory)
     {
         Name = name;
         _logger = loggerFactory?.CreateLogger(GetType()) ?? NullLogger.Instance;
 
-        // Unbounded channel to prevent blocking on writes
-        _messageChannel = Channel.CreateUnbounded<JsonRpcMessage>(new UnboundedChannelOptions
+        // Unbounded channel to prevent blocking on writes. Ensure AutoDetectingClientSessionTransport matches this.
+        _messageChannel = messageChannel ?? Channel.CreateUnbounded<JsonRpcMessage>(new UnboundedChannelOptions
         {
             SingleReader = true,
             SingleWriter = false,
@@ -112,7 +120,7 @@ public abstract partial class TransportBase : ITransport
 
                 case StateConnected:
                     return;
-                
+
                 case StateDisconnected:
                     throw new IOException("Transport is already disconnected and can't be reconnected.");
 

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]

tests/ModelContextProtocol.Tests/Transport/SseClientTransportAutoDetectTests.cs

@@ -0,0 +1,109 @@
+using ModelContextProtocol.Client;
+using ModelContextProtocol.Tests.Utils;
+using System.Net;
+
+namespace ModelContextProtocol.Tests.Transport;
+
+public class SseClientTransportAutoDetectTests(ITestOutputHelper testOutputHelper) : LoggedTest(testOutputHelper)
+{
+    [Fact]
+    public async Task AutoDetectMode_UsesStreamableHttp_WhenServerSupportsIt()
+    {
+        var options = new SseClientTransportOptions
+        {
+            Endpoint = new Uri("http://localhost"),
+            TransportMode = HttpTransportMode.AutoDetect,
+            Name = "AutoDetect test client"
+        };
+
+        using var mockHttpHandler = new MockHttpHandler();
+        using var httpClient = new HttpClient(mockHttpHandler);
+        await using var transport = new SseClientTransport(options, httpClient, LoggerFactory);
+
+        // Simulate successful Streamable HTTP response for initialize
+        mockHttpHandler.RequestHandler = (request) =>
+        {
+            if (request.Method == HttpMethod.Post)
+            {
+                return Task.FromResult(new HttpResponseMessage
+                {
+                    StatusCode = HttpStatusCode.OK,
+                    Content = new StringContent("{\"jsonrpc\":\"2.0\",\"id\":\"init-id\",\"result\":{\"protocolVersion\":\"2024-11-05\",\"capabilities\":{\"tools\":{}}}}"),
+                    Headers =
+                    {
+                        { "Content-Type", "application/json" },
+                        { "mcp-session-id", "test-session" }
+                    }
+                });
+            }
+
+            // Shouldn't reach here for successful Streamable HTTP
+            throw new InvalidOperationException("Unexpected request");
+        };
+
+        await using var session = await transport.ConnectAsync(TestContext.Current.CancellationToken);
+        
+        // The auto-detecting transport should be returned
+        Assert.NotNull(session);
+    }
+
+    [Fact] 
+    public async Task AutoDetectMode_FallsBackToSse_WhenStreamableHttpFails()
+    {
+        var options = new SseClientTransportOptions
+        {
+            Endpoint = new Uri("http://localhost"),
+            TransportMode = HttpTransportMode.AutoDetect,
+            Name = "AutoDetect test client"
+        };
+
+        using var mockHttpHandler = new MockHttpHandler();
+        using var httpClient = new HttpClient(mockHttpHandler);
+        await using var transport = new SseClientTransport(options, httpClient, LoggerFactory);
+
+        var requestCount = 0;
+
+        mockHttpHandler.RequestHandler = (request) =>
+        {
+            requestCount++;
+
+            if (request.Method == HttpMethod.Post && requestCount == 1)
+            {
+                // First POST (Streamable HTTP) fails
+                return Task.FromResult(new HttpResponseMessage
+                {
+                    StatusCode = HttpStatusCode.NotFound,
+                    Content = new StringContent("Streamable HTTP not supported")
+                });
+            }
+
+            if (request.Method == HttpMethod.Get)
+            {
+                // SSE connection request
+                return Task.FromResult(new HttpResponseMessage
+                {
+                    StatusCode = HttpStatusCode.OK,
+                    Content = new StringContent("event: endpoint\r\ndata: /sse-endpoint\r\n\r\n"),
+                    Headers = { { "Content-Type", "text/event-stream" } }
+                });
+            }
+
+            if (request.Method == HttpMethod.Post && requestCount > 1)
+            {
+                // Subsequent POST to SSE endpoint succeeds
+                return Task.FromResult(new HttpResponseMessage
+                {
+                    StatusCode = HttpStatusCode.OK,
+                    Content = new StringContent("accepted")
+                });
+            }
+
+            throw new InvalidOperationException($"Unexpected request: {request.Method}, count: {requestCount}");
+        };
+
+        await using var session = await transport.ConnectAsync(TestContext.Current.CancellationToken);
+        
+        // The auto-detecting transport should be returned
+        Assert.NotNull(session);
+    }
+}

tests/ModelContextProtocol.Tests/Transport/SseClientTransportTests.cs

@@ -17,6 +17,7 @@ public class SseClientTransportTests : LoggedTest
             Endpoint = new Uri("http://localhost:8080"),
             ConnectionTimeout = TimeSpan.FromSeconds(2),
             Name = "Test Server",
+            TransportMode = HttpTransportMode.Sse,
             AdditionalHeaders = new Dictionary<string, string>
             {
                 ["test"] = "header"

tests/ModelContextProtocol.Tests/Transport/StdioClientTransportTests.cs

@@ -1,9 +1,10 @@
 using ModelContextProtocol.Client;
+using ModelContextProtocol.Tests.Utils;
 using System.Runtime.InteropServices;
 
 namespace ModelContextProtocol.Tests.Transport;
 
-public class StdioClientTransportTests
+public class StdioClientTransportTests(ITestOutputHelper testOutputHelper) : LoggedTest(testOutputHelper)
 {
     [Fact]
     public async Task CreateAsync_ValidProcessInvalidServer_Throws()
@@ -11,10 +12,10 @@ public class StdioClientTransportTests
         string id = Guid.NewGuid().ToString("N");
 
         StdioClientTransport transport = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ?
-            new(new() { Command = "cmd", Arguments = ["/C", $"echo \"{id}\" >&2"] }) :
-            new(new() { Command = "ls", Arguments = [id] });
+            new(new() { Command = "cmd", Arguments = ["/C", $"echo \"{id}\" >&2"] }, LoggerFactory) :
+            new(new() { Command = "ls", Arguments = [id] }, LoggerFactory);
 
-        IOException e = await Assert.ThrowsAsync<IOException>(() => McpClientFactory.CreateAsync(transport, cancellationToken: TestContext.Current.CancellationToken));
+        IOException e = await Assert.ThrowsAsync<IOException>(() => McpClientFactory.CreateAsync(transport, loggerFactory: LoggerFactory, cancellationToken: TestContext.Current.CancellationToken));
         Assert.Contains(id, e.ToString());
     }
 }