Add automatic transport fallback for SSE client connections

global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "9.0.100",
+    "version": "8.0.115",
     "rollForward": "minor"
   }
 }

src/ModelContextProtocol/Client/AutoDetectingClientSessionTransport.cs

@@ -0,0 +1,163 @@
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using ModelContextProtocol.Protocol;
+using System.Diagnostics;
+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 DelegatingChannelReader<JsonRpcMessage> _delegatingChannelReader;
+
+    private StreamableHttpClientSessionTransport? _streamableHttpTransport;
+    private SseClientSessionTransport? _sseTransport;
+
+    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;
+        _delegatingChannelReader = new DelegatingChannelReader<JsonRpcMessage>(this);
+    }
+
+    /// <summary>
+    /// Returns the active transport (either StreamableHttp or SSE)
+    /// </summary>
+    internal ITransport? ActiveTransport => _streamableHttpTransport != null ? (ITransport)_streamableHttpTransport : _sseTransport;
+
+    public ChannelReader<JsonRpcMessage> MessageReader => _delegatingChannelReader;
+
+    /// <inheritdoc/>
+    public async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
+    {
+        if (_streamableHttpTransport == null && _sseTransport == null)
+        {
+            var rpcRequest = message as JsonRpcRequest;
+
+            // Try StreamableHttp first
+            _streamableHttpTransport = new StreamableHttpClientSessionTransport(_options, _httpClient, _loggerFactory, _name);
+
+            try
+            {
+                LogAttemptingStreamableHttp(_name);
+                var response = await _streamableHttpTransport.SendInitialRequestAsync(message, cancellationToken).ConfigureAwait(false);
+
+                // If the status code is not success, fall back to SSE
+                if (!response.IsSuccessStatusCode)
+                {
+                    LogStreamableHttpFailed(_name, response.StatusCode);
+
+                    await _streamableHttpTransport.DisposeAsync().ConfigureAwait(false);
+                    await InitializeSseTransportAsync(message, cancellationToken).ConfigureAwait(false);
+                    return;
+                }
+
+                // Process the streamable HTTP response using the transport
+                await _streamableHttpTransport.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
+
+                // Signal that we have established a connection
+                LogUsingStreamableHttp(_name);
+                _delegatingChannelReader.SetConnected();
+            }
+            catch (Exception ex)
+            {
+                LogStreamableHttpException(_name, ex);
+
+                await _streamableHttpTransport.DisposeAsync().ConfigureAwait(false);
+
+                // Propagate the original exception
+                throw;
+            }
+        }
+        else if (_streamableHttpTransport != null)
+        {
+            await _streamableHttpTransport.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
+        }
+        else if (_sseTransport != null)
+        {
+            await _sseTransport.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
+        }
+    }
+
+    private async Task InitializeSseTransportAsync(JsonRpcMessage message, CancellationToken cancellationToken)
+    {
+        _sseTransport = new SseClientSessionTransport(_options, _httpClient, _loggerFactory, _name);
+
+        try
+        {
+            LogAttemptingSSE(_name);
+            await _sseTransport.ConnectAsync(cancellationToken).ConfigureAwait(false);
+            await _sseTransport.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
+
+            // Signal that we have established a connection
+            LogUsingSSE(_name);
+            _delegatingChannelReader.SetConnected();
+        }
+        catch (Exception ex)
+        {
+            LogSSEConnectionFailed(_name, ex);
+            _delegatingChannelReader.SetError(ex);
+            await _sseTransport.DisposeAsync().ConfigureAwait(false);
+            throw;
+        }
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        try
+        {
+            if (_streamableHttpTransport != null)
+            {
+                await _streamableHttpTransport.DisposeAsync().ConfigureAwait(false);
+            }
+
+            if (_sseTransport != null)
+            {
+                await _sseTransport.DisposeAsync().ConfigureAwait(false);
+            }
+        }
+        catch (Exception ex)
+        {
+            LogDisposeFailed(_name, ex);
+        }
+    }
+
+    [LoggerMessage(Level = LogLevel.Debug, Message = "{EndpointName}: Attempting to connect using Streamable HTTP transport.")]
+    private partial void LogAttemptingStreamableHttp(string endpointName);
+
+    [LoggerMessage(Level = LogLevel.Debug, Message = "{EndpointName}: Streamable HTTP transport failed with status code {StatusCode}, falling back to SSE transport.")]
+    private partial void LogStreamableHttpFailed(string endpointName, System.Net.HttpStatusCode statusCode);
+
+    [LoggerMessage(Level = LogLevel.Debug, Message = "{EndpointName}: Streamable HTTP transport failed with exception, falling back to SSE transport.")]
+    private partial void LogStreamableHttpException(string endpointName, Exception exception);
+
+    [LoggerMessage(Level = LogLevel.Debug, 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.Debug, Message = "{EndpointName}: Using SSE transport.")]
+    private partial void LogUsingSSE(string endpointName);
+
+    [LoggerMessage(Level = LogLevel.Error, Message = "{EndpointName}: Failed to connect using both Streamable HTTP and SSE transports.")]
+    private partial void LogSSEConnectionFailed(string endpointName, Exception exception);
+
+    [LoggerMessage(Level = LogLevel.Warning, Message = "{EndpointName}: Error disposing transport.")]
+    private partial void LogDisposeFailed(string endpointName, Exception exception);
+}

src/ModelContextProtocol/Client/DelegatingChannelReader.cs

@@ -0,0 +1,149 @@
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using System.Threading.Channels;
+
+namespace ModelContextProtocol.Client;
+
+/// <summary>
+/// A <see cref="ChannelReader{T}"/> implementation that delegates to another reader
+/// after a connection has been established.
+/// </summary>
+/// <typeparam name="T">The type of data in the channel.</typeparam>
+internal sealed class DelegatingChannelReader<T> : ChannelReader<T>
+{
+    private readonly TaskCompletionSource<bool> _connectionEstablished;
+    private readonly AutoDetectingClientSessionTransport _parent;
+
+    public DelegatingChannelReader(AutoDetectingClientSessionTransport parent)
+    {
+        _parent = parent;
+        _connectionEstablished = new TaskCompletionSource<bool>();
+    }
+
+    /// <summary>
+    /// Signals that the transport has been established and operations can proceed.
+    /// </summary>
+    public void SetConnected()
+    {
+        _connectionEstablished.TrySetResult(true);
+    }
+
+    /// <summary>
+    /// Sets the error if connection couldn't be established.
+    /// </summary>
+    public void SetError(Exception exception)
+    {
+        _connectionEstablished.TrySetException(exception);
+    }
+
+    /// <summary>
+    /// Gets the channel reader to delegate to.
+    /// </summary>
+    private ChannelReader<T> GetReader()
+    {
+        if (_connectionEstablished.Task.Status != TaskStatus.RanToCompletion)
+        {
+            throw new InvalidOperationException("Transport connection not yet established.");
+        }
+
+        return (_parent.ActiveTransport?.MessageReader as ChannelReader<T>)!;
+    }
+
+#if !NETSTANDARD2_0
+    /// <inheritdoc/>
+    public override bool CanCount => GetReader().CanCount;
+
+    /// <inheritdoc/>
+    public override bool CanPeek => GetReader().CanPeek;
+
+    /// <inheritdoc/>
+    public override int Count => GetReader().Count;
+#endif
+
+    /// <inheritdoc/>
+    public override bool TryPeek(out T item)
+    {
+        if (_connectionEstablished.Task.Status != TaskStatus.RanToCompletion)
+        {
+            item = default!;
+            return false;
+        }
+
+        return GetReader().TryPeek(out item!);
+    }
+
+    /// <inheritdoc/>
+    public override bool TryRead(out T item)
+    {
+        if (_connectionEstablished.Task.Status != TaskStatus.RanToCompletion)
+        {
+            item = default!;
+            return false;
+        }
+
+        return GetReader().TryRead(out item!);
+    }
+
+    /// <inheritdoc/>
+    public override ValueTask<bool> WaitToReadAsync(CancellationToken cancellationToken = default)
+    {
+        // First wait for the connection to be established
+        if (_connectionEstablished.Task.Status != TaskStatus.RanToCompletion)
+        {
+            return new ValueTask<bool>(WaitForConnectionAndThenReadAsync(cancellationToken));
+        }
+
+        // Then delegate to the active reader
+        return GetReader().WaitToReadAsync(cancellationToken);
+    }
+
+    private async Task<bool> WaitForConnectionAndThenReadAsync(CancellationToken cancellationToken)
+    {
+        await _connectionEstablished.Task.ConfigureAwait(false);
+        return await GetReader().WaitToReadAsync(cancellationToken).ConfigureAwait(false);
+    }
+
+    /// <inheritdoc/>
+    public override ValueTask<T> ReadAsync(CancellationToken cancellationToken = default)
+    {
+        // First wait for the connection to be established
+        if (_connectionEstablished.Task.Status != TaskStatus.RanToCompletion)
+        {
+            return new ValueTask<T>(WaitForConnectionAndThenGetItemAsync(cancellationToken));
+        }
+
+        // Then delegate to the active reader
+        return GetReader().ReadAsync(cancellationToken);
+    }
+
+    private async Task<T> WaitForConnectionAndThenGetItemAsync(CancellationToken cancellationToken)
+    {
+        await _connectionEstablished.Task.ConfigureAwait(false);
+        return await GetReader().ReadAsync(cancellationToken).ConfigureAwait(false);
+    }
+
+#if NETSTANDARD2_0
+    public IAsyncEnumerable<T> ReadAllAsync(CancellationToken cancellationToken = default)
+    {
+        // Create a simple async enumerable implementation
+        async IAsyncEnumerable<T> ReadAllAsyncImplementation()
+        {
+            while (await WaitToReadAsync(cancellationToken).ConfigureAwait(false))
+            {
+                while (TryRead(out var item))
+                {
+                    yield return item;
+                }
+            }
+        }
+
+        return ReadAllAsyncImplementation();
+    }
+#else
+    /// <inheritdoc/>
+    public override IAsyncEnumerable<T> ReadAllAsync(CancellationToken cancellationToken = default)
+    {
+        return base.ReadAllAsync(cancellationToken);
+    }
+#endif
+}

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/SseClientTransport.cs

@@ -57,11 +57,24 @@ public SseClientTransport(SseClientTransportOptions transportOptions, HttpClient
     /// <inheritdoc />
     public async Task<ITransport> ConnectAsync(CancellationToken cancellationToken = default)
     {
-        if (_options.UseStreamableHttp)
+        switch (_options.TransportMode)
         {
-            return new StreamableHttpClientSessionTransport(_options, _httpClient, _loggerFactory, Name);
+            default:
+                throw new ArgumentException($"Unsupported transport mode: {_options.TransportMode}", nameof(_options.TransportMode));
+
+            case HttpTransportMode.AutoDetect:
+                return new AutoDetectingClientSessionTransport(_options, _httpClient, _loggerFactory, Name);
+
+            case HttpTransportMode.StreamableHttp:
+                return new StreamableHttpClientSessionTransport(_options, _httpClient, _loggerFactory, Name);
+
+            case HttpTransportMode.Sse:
+                return await ConnectSseTransportAsync(cancellationToken).ConfigureAwait(false);
         }
+    }
 
+    private async Task<ITransport> ConnectSseTransportAsync(CancellationToken cancellationToken)
+    {
         var sessionTransport = new SseClientSessionTransport(_options, _httpClient, _loggerFactory, Name);
 
         try